ABSTRACTGamma‐glutamyl transpeptidase (γ‐GT) activity was measured in a homogenate of penile foreskin, usingγ‐glutamyl‐p‐nitroanilide as a substrate. It was found thatγ‐GT activity was present in both the epidermis and the dermis, being more active in the former.The optimum pH for the enzyme was 8.5–9.0 in Tris buffer, which is similar to that for the rat kidney and human serum enzymes. It was also revealed that glycylglycine was the most effective activator of the enzyme. Some activation was also observed in the presence of L‐glutamine. But L‐methionine, L‐homoserine, L‐glutamic acid, L‐arginine, L‐aspartic acid, glycine and L‐valine inhibited the activity, suggesting that these amino acids do not act as acceptors of theγ‐glutamyl moiety. The enzyme was remarkably inhibited by bromsulphalein, oxidized glutathione, and by L‐serine in the presence of borate. These inhibitions were more severe than thoes observed with