Puromycin blocks protein synthesis in vitro and in vivo and is a useful agent in demonstrating the role of protein synthesis in acute physiological response. Its action appears to occur at the stage of formation of the polypeptide chain at the ribosomal site. The transfer of the amino acids from sRNA into ribosomal protein is inhibited in the presence of puromnycin, and a subsequent release of incomplete, soluble protein from the ribosome then takes place without ribosomnal breakdown. Maximum inhibition of protein synthesis is obtained with puromycin analogs that contain an aromatic amino acid attached to the 3' amino group. Although the specificity of this antibiotic for the amino acid portion of the muolecule is an established fact, the reasons for this specificity remain obscure, and current concepts include a direct reaction with the enzyme system and a metabolic analogue response. Because of this inhibition of protein synthesis, puromycin is a useful agent for metabolic studies involving either enzymatic or hormonal systems.