A competition-based immunoassay that once set the standard for sensitivity: a sample’s unlabeled target vies with a radioactively tagged version for antibody binding sites, and the degree to which binding is blocked reveals the concentration against a calibration curve. It let food analysts detect hormones, toxins, allergens, and drug residues at trace levels long before modern mass spectrometry. Radioactive handling requirements have since pushed many labs toward enzyme or fluorescence labels, but the principle still underpins today’s non-radioactive immunoassays. Modern laboratories increasingly replace isotopic labels with enzymatic or fluorescent ones.