The peptide composition and antigenic cross-reactivity of partially purified and concentrated haemolysins of 16 strains of Vibrio vulnificus were examined by SDS-PAGE and immunoblotting analysis, using a monoclonal antibody (MAb), 6F8D, raised against the haemolysin. All strains produced a common peptide of 36 kDa and the MAb reacted with this peptide. In some strains, larger molecules, including a 56 kDa peptide, were produced, but the MAb did not react with this peptide. The haemolytic activity of the strains was effectively neutralized by the MAb, except in the case of strains producing the 56 kDa peptide. These findings indicate that the 36 kDa haemolysin is common to all 16 strains and that V. vulnificus can produce a second haemolysin which differs in molecular mass and antigenicity.