Aserine protease has been purified and characterized from Periserrula leucophryna using a combination of ammonium sulphate fractionation, gel filtration, ion exchange and Benzamidine-Sepharose chromatography. Analysis of the purified enzyme with SDS-PAGE and gel filtration revealed a single polypeptide chain with an apparent molecular weight of 28 kDa. The proteolytic activity was stable up to 45 degreesC but became unstable over 50 degreesC. The enzyme had an optimum pH of 10 and maintained its stability over a broad range of pH between 4 and 12. Ca2- was not required for enzyme activity, and several heavy metals such as Cd2+ and Hg2+ had no effect on protease activity. Treatment with sodium dodecyl sulphate did not inactivate the enzyme, which retained approximately 50% of its original activity, even in the presence of 5% SDS. Furthermore, enzyme activity was not influenced by the presence of either reducing or oxidizing agents. According to inhibition profiles obtained with several serine protease inhibitors such as leupeptin and PMSF, it was confirmed that the purified protease belongs to the family of serine proteases.