An extracellular proteinase from a fast-growing strain of Hendersonula toruloidea was demonstrated when it was cultivated in liquid medium containing bovine serum albumin as the sole nitrogen source. Purification to homogeneity of the proteinase was performed by carboxymethyl cellulose, CM Sephadex G-50, and Sephacryl S-200 column chromatographies. The purified enzyme was a chymotrypsinlike serine proteinase, as indicated by the strong inhibitory activities of diisopropyl fluorophosphate, phenylmethylsulfonyl fluoride, L-1-tosylamido-2-phenylethylchloromethyl ketone, and chymostatin and good kinetic constants for a synthetic substrate, Suc-Ala-Ala-Pro-Phe-MCA. The enzyme had a pI of 8.4, a pH optimum of 9.0, and a molecular weight of 34,000. Skin constituents such as stratum corneum and nail, but not hair, were easily digested by this enzyme. Thus, this extracellular proteinase may play a role in the invasion of thickly keratinized skin and nail by this organism.