The results of studies described in this report support the ideas that virulence of Vibrio vulnificus is associated, at least in part, with resistance to phagocytosis and that the ability of the bacterium to resist phagocytosis results from its possession of an antiphagocytic surface antigen. These conclusions are based on the observations that (1) human polymorphonuclear leukocytes (PMNLs) exhibited a significantly weaker chemiluminescent response and phagocytic response when interacting with a virulent strain of the bacterium than when challenged with a weakly virulent strain of the bacterium; (2) rabbit antiserum to the virulent bacterium, but not normal rabbit serum, significantly enhanced the chemiluminescent response of PMNLs challenged with the virulent bacterium and significantly enhanced ingestion of the bacterium by the PMNLs; and (3) the opsonic activity of the antiserum was removed by adsorption with a formalin-killed, whole cell preparation of the virulent bacterium.