To evaluate the role of capsular polysaccharide (CPS) as a virulence factor, the interaction of V. vulnificus with mouse peritoneal macrophages and serum, which are involved in the clearance of bacteria from blood and other tissues, were examined. In this study, MO6-24/0 (wild strain; hemolysin- and capsule-positive), MO6-24/I' (acapsular spontaneous mutant), CVD 752 (acapsular transposon mutant), and CVD 707 (hemolysin-negative and capsule-positive mutant) were used. The strain with CPS (MO6-24/0 and CVD 707) were more resistant to phagocytosis by mouse peritoneal macrophages compared with acapsular strains (MO6-24/T and CVD 752), and the resistance to phagocytosis was not changed by serum opsonin in the capsular strains. Acapsular strains were more susceptible to serum bactericidal activity than the capsular strains through the classical complement pathway. MO6-24/0 strain were detected in blood, spleen, liver and lung at 4 hours after intraperitoneally infection, whereas CVD 752 were not detected. All tested strains could induced the transcription of inflammatory cytokine gene such as IL-1, IL-6, IL-10 and TNF-u, and their inductions were not decreased by cytochalasin B treatment. This results demonstrate that CPS of V. vulnificus plays an important role in V. vulnificus infection through interfering nonspecific host defense system such as blood clearance and phagocytosis.