A preparation of pure type A toxin of Clostridium botulinum was labeled with 131I in the presence of chloramine-T and carrier-free isotope. The radioactive toxin (131I Tox) was used in a radioimmunoassay procedure similar to that of Berson and Yalow. Dilutions of antibody to the toxin, capable of binding 50% of the added 131I Tox, were mixed with a sample of the labeled toxin and various concentrations of unlabeled toxin (‚Cold’ Tox). When concentration of Cold Tox were plotted against the ratio 131I bound/131I not bound, a standard curve was established that could be used to estimate the concentration of Cold Tox in a test mixture. This assay was sensitive to as little as 100 mouse minimum lethal dose and was highly specific for the serological type of the toxin used. Images