Inhibitor studies with peptide substrates demonstrate that bovine lens neutral proteinase comprises three distinct activities. Diisopropylfluoro-phosphate distinguishes the activity hydrolyzing carbobenzoxy-Gly-Gly-Leu-p-nitroanilide (inhibited) from that hydrolyzing carbobenzoxy-Leu-Leu-Glu-2-naphthylamide (not inhibited). Leupeptin inhibits hydrolysis of the substrate carbobenzoxy-Leu-Leu-Arg-2-naphthylamide, but not hydrolysis of car-bobenzoxy-Gly-Gly-Leu-p-nitroanilide or carbobenz-oxy-Leu-Leu-Glu-2-naphthylamide, demonstrating the presence of the third activity. Inhibition of the three activities by thiol reagents suggests that each activity may be dependent on an active-site cysteine residue.