Exocellular proteases produced by Vibrio fluvialis, V furnissii, V metschnikovii and V campbellii were characterized and compared to those of V mimicus protease (VMP) and V vulnificus protease (VVP). These proteases possessed both elastolytic and hemagglutinating abilities and were identified, except that of V. metschnikovii, as metalloprotease. Conversely, V. metschnikovii protease failed to exhibit some of the salient features for metalloproteases suggesting the existence of protease(s) other than metalloprotease. However, antibodies against VVP cross-reacted to these proteases and to VMP indicating antigenic relatedness amongst vibrio proteases. This study, thus, demonstrated the prevalent distributions of antigenically related proteases both in pathogenic and non-pathogenic vibrios, bringing their status as a virulence determinant into question.Key words: Vibrio, Protease, Hemagglutinin Exocellular proteases are established as virulence determinants in some bacteria (7). Of these, the protease from Vibrio cholerae (5) is well known for its roles in the pathogenesis of deadly cholera (2, 4). This protease was first discovered as an adhesion factor due to its hemagglutinating (HA) abilities (6), and was then shown to be a protease (5) possessing the ability to nick and activate the cholera enterotoxin (2). Later, it was demonstrated to be a metalloprotease containing zinc as an essential ligand (1). Similarly, proteases from a number of pathogenic vibrios were also shown to be metalloproteases (3,(8)(9)(10)14). Conversely, the protease from a hyperproducing mutant of the pathogen V parahaemolyticus was shown to be a serine protease (Hata et al, personal communication). However, the nature of proteases from a number of important human pathogenic Vibrio spp. remain unknown.Several investigators found definite correlations between the in vitro protease productions and vibrio infections (11,14), while others have not (7). In any case, studies on the comparative abilities of pathogenic and non-pathogenic vibrios to produce protease have not been performed yet.The zinc-metalloprotease produced by the fish pathogen V. anguillarum was first reported to share several properties with those of V vulnificus and V cholerae (7). Similarly, immunological cross-reactions between the metalloproteases from V. mimicus and V. cholerae are established (3). Recently, a high degree of homology in the N-terminal amino acid sequences among a number of vibrio metalloproteases has been documented (7). However, studies on the comparative biochemical and immunological aspects of vibrio proteases have not been attempted. In this communication, we report the production of antigenically related proteases possessing the HA abilities similar to some of the pathogenic as well as non-pathogenic vibrios. Furthermore, we report the characterizations and comparisons of those proteases to the metalloproteases from V. vulnificus and V mimicus.The four Vibrio species used for the production and characterization of proteases were V fluvialis ...