1. A comparative study on the mode of action of two highly purified acid endopeptidases (EC 3.4.-) from Aspergillus niger var. macrosporus, acid proteinase A and B, on the B-chain of performic acid oxidized insulin was performed, putting emphasis on the quantitative analysis of the effects of enzyme A. Acid proteinase A behaved very specifically towards the substrate and hydrolyzed four peptide bonds exclusively: three major sites, where hydrolysis proceeded rapidly and almost completely, Asn3-Gln4, Glu13-Ala14, and Tyr26-Thr27; and a minor one, Gly20-Glu21, at which hydrolysis was much slower. 2. The effects of four protease inhibitors, pepstatin, diazoacetyl-D,L-norleucine methyl ester/Cu(II), di-isopropyl phosphorofluoridate, and 1,2-epoxy-3-(p-nitrophenozy) propane on acid proteinases A and B were studied. Acid proteinase A preparations, treated with the former two inhibitors, were used to establish that the major sites of attack were really affected by enzyme A and not by contaminating proteinase B.