A nonspecific binding reaction between cyanic acid formed from urea and protein or peptide is called carbamylation. In the present study, insulin, a peptide hormone, was subjected to carbamylation and the activity of carbamylated insulin was determined. Both immunological and biological activities of insulin changed on carbamylation. The decrease in biological activity in respect to glucose oxidation of fat cells or receptor-binding capacity of rat hepatocytes was greater than that in immunological activity.