Aspergillus fumigatus secreted an inducible alkaline protease (AlPase) when cultivated in the presence of collagen (200 micrograms/ml) as sole nitrogen and carbon source. Proteolytic activity was maximum at pH 9.0 with azocollagen as substrate. The enzyme, which was the major protein found in the supernate of a liquid culture, was purified by ammonium sulphate precipitation and gel filtration. The Mr was determined to be 33 Kda by gel filtration and sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The isoelectric point was estimated to be pH 8.2. Divalent cations strongly inhibited enzyme activity, whereas non-ionic detergents and reducing agents had no effect. A. fumigatus AlPase was totally inhibited by phenylmethanesulphonyl fluoride, antipain, chymostatin and alpha-2-macroglobulin. A. fumigatus AlPase is closely related to the A. oryzae AlPase, a serine protease of the subtilisin family, as attested by the antigen pattern seen by immunoblotting. The high collagenic activity and the ability of A. fumigatus AlPase to digest elastin could play a role in the invasion of the tissues by the fungus.