A zinc metalloprotease (AP34) from Aeromonas caviae was purified by ammonium sulfate precipitation and subsequent gel filtration through Sephadex G-100 and Sephadex G-50 Superfine. The molecular mass was estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to be 34 kDa. The protease showed maximum activity at pH 7.0 and was stable at 60 degrees C. AP34 was completely inactivated by EDTA and Zincov. The N-terminal amino acid sequence of AP34 showed a high degree of homology with a range of proteases within the family Vibrionaceae, including the hemagglutinin/protease (HA/P) of Vibrio cholerae. Immunologic relatedness of AP34 and HA/P was demonstrated by Western blotting. AP34-like protease was widely distributed among the aeromonad strains.