An aspartic proteinase (PEP) from the culture supernatant of a clinical isolate of Aspergillus fumigatus was purified to virtual homogeneity at a yield of 24%. The procedure involved affinity chromatography on pepstatin agarose, the interaction requiring a chaotropic salt (sodium trifluoroacetate) for complete elution of the enzyme. Among 11 amino acids of the N-terminal region, nine were identical with the corresponding sequence of the aspartic proteinase aspergillopepsin A from Aspergillus niger var. awamori (previously called Aspergillus awamori). Thus PEP belongs to the aspergillopepsins, a family of closely related aspartic proteinases produced by aspergilli. Specific antibodies against PEP were detected by dot blot assay in sera of two patients with aspergillosis. In addition, PEP antigen was demonstrated by immunofluorescence in mycotic human lung, using specifically elicited antibodies from guinea-pigs. PEP had an estimated molecular mass of 38 kDa and the pI was determined at pH 4.2. PEP is therefore likely to be closely related to an acid proteinase of A. fumigatus which was originally described in 1981.