A fibrinogenolytic proteinase has been isolated from Aspergillus fumigatus culture filtrate by ammonium sulfate precipitation followed by successive chromatographies on Sephadex G-75 and immobilized phenylalanine. The purified proteinase exhibited a molecular weight of about 33 kDa. When analysed by SDS-polyacrylamide gels containing co-polymerized fibrinogen, the proteinase appeared as a broad band at the top of the gels, which could correspond to polymerization of the enzyme, as suggested by SDS-PAGE analysis of the unboiled eluate. The isoelectric point was 8.75 and the enzyme was not glycosylated. Proteinase activity was optimum at pH 9 and between 37 and 42 degrees C, although a decrease in activity was observed above 37 degrees C. PMSF and chymostatin markedly inhibited the proteinase activity, and good kinetic constants were obtained for the synthetic substrate, N-Suc-Ala-Ala-Pro-Phe-pNA. These results provide direct evidence that this enzyme belongs to the chymotrypsin-like serine proteinase group.