Tracer I131 insulin combines with its specific antibody obtained from immunized guinea pigs to form a soluble complex which is precipitated by anti-guinea pig serum obtained from immunized rabbits. The change in percent of tracer precipitated, as increasing amounts of unlabeled insulin are added, forms the basis of this assay. Dilutions of anti-insulin serum of 1:5000 to 1:40000 were suitable for assaying 1[mu]U to 100[mu]U of insulin. As much as 16[mu]U of I131 insulin can be used as a tracer, when assaying as little as 1[mu]U of insulin. Therefore, Abbott Laboratories'' I131 insulin of 20-30 mc/mg specific activity can be used. Preliminary insulin assay values are presented for human plasma during a glucose tolerance test and for microdissected rat islet tissue.