Human islet cell tumor tissue and isolated islets of Langerhans from rats incorporated radioactive amino acids in vitro into insulin and a larger acid-alcohol soluble protein which could be separated from insulin by gel filtration. The amino acids were incorporated into the larger protein earlier than into insulin; only after incubation of islets for approximately 30 minutes did radioactivity begin to appear in insulin. The transfer of about 70 percent of the radioactivity of the larger protein to insulin was demonstrated in the absence of new peptide bond synthesis (cycloheximide), or during incubation with unlabeled amino acid (chase). The results indicate that the larger protein is a precursor in the biosynthesis of insulin. The name "proinsulin" is suggested for this protein.