Schizophyllum commune produces a variety of mycelial proteolytic enzymes. The specific functions of many of these enzymes are unknown, but several have elevated activity when the mycelium is grown in nitrogen-limiting conditions, suggesting a role in mycelial autolysis. We have purified one of these nitrogen-limitation induced enzymes, a small serine protease, ScPrI, from S. commune mycelial extracts. ScPrI has an apparent molecular mass of 22 kDa and is active against classical substrates for chymotrypsin and subtilisin proteases. The pH optimum for activity is neutral to slightly alkaline and the protein denatures above 50 °C. The enzyme is inhibited by PMSF, TPCK and chymostatin, and it shows little dependence on metal ions. Hydrolysis of oxidized insulin B-chain peptide by ScPrI demonstrated cleavage following aromatic amino acids and leucine. Kinetic analysis of hydrolysis of N- succinyl-AAPF-pNA and N-succinyl-AAPL-pNA revealed similar Kms for both substrates but the Vmax was nearly 3-fold higher for the substrate with phenylalanine in the P1 position.