Candida albicans was able to produce a keratinolytic proteinase (KPase) when cultivated in a medium containing human stratum corneum as a nitrogen source. The KPase was purified to 108.5-fold by ion-exchange chromatography and gel filtration. The molecular weight of the enzyme was estimated to be 42,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and gel filtration through Sephacryl S-200, while the isoelectric point was determined to be at pH 4.5. The enzyme had an optimum pH of 4.0 and was "inactive" below pH 2.5 and above pH 6.0. The activity of KPase after preincubation at various temperatures was stable up to 50 degrees C. The keratinolytic activity was not affected by the addition of nonionic detergents and divalent cations. The enzyme was a glycoprotein and contained a high content of aspartic acid residues (172/1000). Pepstatin and chymostatin inhibited the activity in a dose-dependent manner; however, neither the other group specific inhibitors tested nor the pepsin specific inhibitors, DAN or EPNP, showed any effect on the enzyme. From these inhibitory profiles, this enzyme was determined to be a carboxyl proteinase such as cathepsin D. Among the various substrates for proteolytic enzymes, KPase digested human stratum corneum as much as albumin and hemoglobin. In the three fractions (water soluble, keratin filamentous, and membranous) prepared from human stratum corneum, the keratin filamentous fraction was more susceptible to degradation by KPase than the other two fractions were. KPase also digested much less human fingernail (13%) than human stratum corneum, but did not show any signs of there being any digestion of human scalp hair. These studies suggest that KPase from C. albicans may play an important role in superficial infection by affecting the human stratum corneum of the skin and nail.