RIA (radioimmunoassay) is a competitive immunoassay in which a radiolabelled tracer antigen — almost always an I-125-labelled peptide — competes with unlabelled analyte in the sample for a limited quantity of antibody. Bound and free fractions are separated, and bound radioactivity is counted on a gamma counter. Signal falls as analyte concentration rises, so the standard curve is inverse.
Why it still exists
RIA was developed by Rosalyn Yalow and Solomon Berson for insulin measurement and earned Yalow a share of the 1977 Nobel Prize; it was the first assay sensitive enough to quantify circulating hormones. ELISA has displaced it in most laboratories because it avoids radioactive licensing, waste handling and the roughly 60-day half-life of I-125. RIA persists where it still wins: very small peptides and steroids where sensitivity in the low picogram range matters, and matrices where enzymatic or coloured interferents disturb an optical readout.
Practical constraints are regulatory as much as technical — a radioactive materials licence, dedicated counting equipment, tracer that decays on a fixed schedule, and disposal obligations. Those costs, not assay performance, usually decide the choice.
Related terms and material
immunogen · antiserum · species reactivity. Reference material: assay kits, labeled peptides. Further reading: how ELISA and RIA kits quantify peptides.