Unsealed tracers

Feasibility review

Iodine-125 for research

A high specific activity label for proteins, peptides and receptor ligands, counted in a gamma counter.

Half-life: 59.41 days
Main gamma: 35.5 keV
Decay: Electron capture to Te-125
Exempt quantity: 1 µCi

Request an I-125 feasibility review How ordering works

I-125 at a glanceIAEA evaluated data

a What a germanium detector records, simulated from the IAEA lines. b Activity left after the reference date. Data: IAEA [1]

Research applications

Where I-125 does the work.

Figures are simulated examples of each measurement.

Protein chemistry
Labeling proteins and peptides^{2, 3}
Sodium [¹²⁵I]iodide is attached to tyrosines by oxidation, or to amino groups through a pre-labeled acylating reagent.
More: Labeling proteins and peptides
A small gel filtration column then separates the labeled protein, which comes off first, from free iodide. Typically about 70 percent of the iodine ends up on the protein.
One iodine-125 atom per molecule gives about 2,200 Ci/mmol, roughly 75 times one tritium atom.
Pharmacology
Receptor binding^{4, 5, 6}
The high specific activity of ¹²⁵I ligands lets a few micrograms of membrane protein per tube measure picomolar binding.
More: Receptor binding
[¹²⁵I]iodocyanopindolol binds β-adrenoceptors with dissociation constants of 27 to 40 pM, measured with 3 µg of protein per assay.
A subtype-selective competitor gives a two-step curve where both subtypes are present. The Cheng-Prusoff equation converts its IC₅₀ to an inhibition constant.
Neuroscience
Receptor autoradiography⁵
Sections labeled with an ¹²⁵I ligand map receptor density across the brain after a day or less on film.
More: Receptor autoradiography
With a β1-selective blocker added, the same sections show that β1 receptors make up 88 percent of β-adrenergic sites in cingulate cortex but only 9 percent in the cerebellar molecular layer.
Immunoassay
Radioimmunoassay⁷
Labeled and unlabeled antigen compete for a limited amount of antibody, so bound label falls as unlabeled antigen rises.
More: Radioimmunoassay
Known standards give a curve that converts bound label to concentration. Yalow reported gastrin measurable at 0.1 pg/ml, about 0.05 pM.
Protein analysis
Western blots^{8, 9}
¹²⁵I-labeled protein A finds antibody-bound proteins on a blot, for film or direct counting.
More: Western blots
The paper that named western blotting detected its bands this way. In one quantitative method, band radioactivity was linear from 0.1 to 5 µg of protein.
Radioactivity counting
Gamma counting^{1, 2}
Iodine-125 is counted directly in a gamma counter, with no scintillation cocktail, at about 80 percent efficiency.
More: Gamma counting
The detector sees its tellurium X-rays at 27 to 31 keV and its 35.5 keV gamma ray.

Forms

I-125 forms researchers ask for

Every form is confirmed with its manufacturer before you order.

Sodium [¹²⁵I]iodide
Iodide in alkaline solution
Labeling proteins and peptides
Feasibility review
Labeled protein or peptide
Iodine-125 labeling of a protein or peptide you specify
Binding assays and immunoassays
Feasibility review
¹²⁵I-labeled ligand
Receptor ligand labeled with iodine-125, in solution
Receptor binding and autoradiography
Feasibility review

Activity, certificate and lead time are set with the manufacturer at quote.

Before you order

Plan for decay and licensing.

Plan around decay

I-125 has a half-life of 59.41 days and loses about 7.8 percent of its activity each week. Ask for the activity you need on the day you start using it.

Activity remaining after

1 day98.8%
1 week92.2%
2 weeks84.9%
30 days70.5%

Licensing

Exempt quantityNeeds a license1 µCi

Activity of one vial, log scale

A single quantity of I-125 at or below 1 µCi is an exempt quantity under 10 CFR 30.71, Schedule B. Above that, your institution needs a license that covers it.

Combining exempt quantities to exceed the limit is not covered.
Thin lead stops its photons: the half-value layer in lead is about 0.02 mm, and 0.15 mm of lead foil lets through about 1 percent.
Acidic and frozen solutions turn iodide into volatile iodine, so iodinations run in an approved exhaust hood. Survey with a sodium iodide probe, about 13 percent efficient, rather than a Geiger-Mueller probe at about 0.5 percent.
Agreement States apply equivalent rules. Your RSO reviews the exact material before you order.

Sources: 10, 11, 12

Start a request

Request a feasibility review for I-125.

No license file needed to ask. A scientist reads every request and replies with the next step.

Name the moleculeGive the protein, peptide or ligand, the labeling position if it matters and the amount.
Time the labelingActivity falls about 8 percent a week, so plan the work soon after delivery.
Prepare a hoodIodinations run in an approved exhaust hood with lead shielding ready.

Decay data

I-125 decay data

Decay schemeI-125 to Te-125

Energies in keV, with emission probability per decay. Levels not to scale. Data: IAEA [1]

Half-life: 59.407 days ± 0.01
Decay mode: Electron capture (100%)
Daughter: Te-125
Decay energy (Q): 185.8 keV

I-125 emissions
Radiation	Energy (keV)	Per 100 decays
Gamma	35.49	6.68
X-ray	4.13	14.8
X-ray	27.2	39.4
X-ray	27.47	73.4
X-ray	31.09	21
X-ray	31.36	25.6

Source: IAEA LiveChart of Nuclides, ENSDF evaluation by J. KATAKURA (literature cutoff 1 January 2010), retrieved 25 September 2026 [1]. Reference values; a manufacturer's certificate states the activity of a specific lot.

Questions, answered

I-125 questions

How long does iodine-125 last?

Its half-life is 59.41 days, so it loses about 8 percent a week and about a third remains after three months.

Can we buy iodine-125 without a radioactive materials license?

Only in very small amounts. A single quantity of I-125 at or below 1 microcurie is an exempt quantity under 10 CFR 30.71. Labeling work uses more and needs a license that covers it. Your RSO decides how your institution handles each case.

How is iodine-125 shielded?

With thin lead. Its half-value layer in lead is about 0.02 mm, and 0.15 mm of lead foil lets through about 1 percent of its photons.

Why do iodinations run in a fume hood?

Acidic and frozen solutions turn iodide into volatile iodine. Keep solutions from turning acidic, and run labeling in an approved exhaust hood with the vial vented through charcoal.

What specific activity can iodine-125 give?

About 2,200 Ci/mmol for one iodine-125 atom per molecule, roughly 75 times one tritium atom.

Does the certificate state the activity on delivery?

It states the activity at the manufacturer's reference date. Iodine-125 loses about 8 percent a week, so check that date and plan the labeling soon after delivery.

More isotopes

Carbon-14 for research
C-14 · Half-life 5,700 years
The label that sits in a molecule's carbon backbone, for following carbon through cells, animals, plants and soil.
Phosphorus-32 for research
P-32 · Half-life 14.27 days
An energetic beta label for DNA and RNA probes, kinase assays and gel shift experiments.
Sulfur-35 for research
S-35 · Half-life 87.37 days
The label for newly made proteins, G protein activation and sharp autoradiographs.
Tritium (H-3) for research
H-3 · Half-life 12.32 years
A low-energy beta label that leaves a molecule almost unchanged, for binding, cell and metabolism studies.
Americium-241 for research
Am-241 · Half-life 432.6 years
Alpha particles near 5.49 MeV and a 59.5 keV gamma ray, from one long-lived source.
Barium-133 for research
Ba-133 · Half-life 10.55 years
Gamma lines from 81 to 384 keV and cesium X-rays near 31 keV: a standard for low- and mid-energy calibration.

Sources

References

IAEA Nuclear Data Section. LiveChart of Nuclides, I-125 decay data (ENSDF evaluation). Retrieved 25 September 2026. www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html
Marek A, Brož B, Kriegelstein M, et al. Late-stage labeling of diverse peptides and proteins with iodine-125. J Pharm Anal. 2025;15(7):101198. doi.org/10.1016/j.jpha.2025.101198
Henriksen-Lacey M, Bramwell V, Perrie Y. Radiolabelling of antigen and liposomes for vaccine biodistribution studies. Pharmaceutics. 2010;2(2):91-104. doi.org/10.3390/pharmaceutics2020091
Engel G, Hoyer D, Berthold R, Wagner H. (±)[125Iodo]cyanopindolol, a new ligand for β-adrenoceptors: identification and quantitation of subclasses of β-adrenoceptors in guinea pig. Naunyn Schmiedebergs Arch Pharmacol. 1981;317(4):277-285. doi.org/10.1007/BF00501307
Rainbow TC, Parsons B, Wolfe BB. Quantitative autoradiography of beta 1- and beta 2-adrenergic receptors in rat brain. Proc Natl Acad Sci U S A. 1984;81(5):1585-1589. doi.org/10.1073/pnas.81.5.1585
Cheng Y, Prusoff WH. Relationship between the inhibition constant (KI) and the concentration of inhibitor which causes 50 per cent inhibition (I50) of an enzymatic reaction. Biochem Pharmacol. 1973;22(23):3099-3108. doi.org/10.1016/0006-2952(73)90196-2
Yalow RS. Radioimmunoassay: a probe for the fine structure of biologic systems. Nobel Lecture, 8 December 1977. Also published in Science. 1978;200(4347):1236-1245. www.nobelprize.org/uploads/2018/06/yalow-lecture.pdf
Burnette WN. "Western blotting": electrophoretic transfer of proteins from sodium dodecyl sulfate-polyacrylamide gels to unmodified nitrocellulose and radiographic detection with antibody and radioiodinated protein A. Anal Biochem. 1981;112(2):195-203. doi.org/10.1016/0003-2697(81)90281-5
Hathaway DR, Haeberle JR. A radioimmunoblotting method for measuring myosin light chain phosphorylation levels in smooth muscle. Am J Physiol. 1985;249(3):C345-C351. doi.org/10.1152/ajpcell.1985.249.3.C345
U.S. Nuclear Regulatory Commission. 10 CFR 30.71, Schedule B (exempt quantities). eCFR, current as of 23 September 2026. www.ecfr.gov/current/title-10/section-30.71
U.S. Nuclear Regulatory Commission. 10 CFR 30.18, Exempt quantities. eCFR, current as of 23 September 2026. www.ecfr.gov/current/title-10/section-30.18
University of Michigan Environment, Health and Safety. Radiation Safety Service: iodine-125. Revision 1, 2020. ehs.umich.edu/wp-content/uploads/2016/04/Iodine-125.pdf

Technical review by ConductScience, 25 September 2026. Next review 25 September 2027.

Need I-125 for a specific instrument or measurement?

Start with what you know. Your RSO and supplier documents come after review. See all radioactive isotopes.

Start a request

Iodine-125 for research

Labeling proteins and peptides2, 3

Receptor binding4, 5, 6

Receptor autoradiography5

Radioimmunoassay7

Western blots8, 9

Gamma counting1, 2

Sodium [¹²⁵I]iodide

Labeled protein or peptide

¹²⁵I-labeled ligand

Plan around decay

Licensing

Carbon-14 for research

Phosphorus-32 for research

Sulfur-35 for research

Tritium (H-3) for research

Americium-241 for research

Barium-133 for research

Need I-125 for a specific instrument or measurement?

Labeling proteins and peptides^{2, 3}

Receptor binding^{4, 5, 6}

Receptor autoradiography⁵

Radioimmunoassay⁷

Western blots^{8, 9}

Gamma counting^{1, 2}