A 23-kDa protein with elastinolytic activity was purified from Aspergillus flavus (NRRL 18543) culture filtrates by gel-filtration chromatography. Severe inhibition of the elastinolytic activity by 1,10-phenanthrolene (5 mM) and EDTA (0.8 mM) indicated that the protein belongs to the metallo class of pro teases. The isoelectric point was 9.0. Natural substrates susceptible to cleavage by this protease, in addition to elastin, included cottonseed storage protein, collagen, ovalbumin and bovine serum albumin. The 23-kDa protein was thermostable to 70 degrees C and retained its elastinolytic activity in concentrated form at 4 degrees C for 6 months. Elastinolytic activity was initially secreted into the culture medium as a 35-kDa protein, which was subsequently converted to a 23-kDa protein, presumably through autolysis. This putative proteolytic degradation product appears to be identical to the 23-kDa protein recovered from the gel-filtration column. The 23-kDa protease may confer selective advantage to the fungus in the extracellular environment because of its temperature and pH stability and wide range of potential natural protein substrates.