Barley peptide hydrolase A acting on N-benzoyl-dl-arginine-p-nitroanilide as substrate is inhibited by the sulphydryl reagents N-ethyl maleimide and p-chloro-mercuriphenyl sulphonate. Oxidized glutathione, however, is non-inhibitory and several sulphydryl compounds are not stimulatory. Di-isopropylphosphofluoridate and 8-hydroxyquinoline also are effective inhibitors. The results are consistent with the presence of one or more exposed serine residues and a metal cation in the active centre, and one or more sulphydryl groups near the active centre of the enzyme.