An immunochemical assay has been designed that utilizes the catalytic properties of glutathione-activated ficin to separate antibody-bound radioinsulin from unbound insulin I-131. Both the recovery of human insulin added to serum and the failure to detect chemically inactivated insulin provide evidence for the precision and specificity of the procedure. Measurements of endogenous insulin in serum (diluted 1:10) made after the administration of glucose or tolbutamide revealed rapid increases in circulating levels of hormone in nondiabetic subjects. The successful application of enzyme hydrolysis as the basis of a reliable and rapid immunoassay for insulin suggests that with the appropriate antibody and labeled antigen, the same principle could be employed to measure concentrations of other hormones or substances that are potentially antigenic.