Vibrio vulnificus causes fatal septicemia in human hosts, which is the consequence of raw shellfish consumption. The mortality following septicemia is dependent on the in vivo production of inflammatory mediators, including tumor necrosis factor-alpha (TNFalpha). The present study was set up to investigate the association of quorum sensing in V. vulnificus with the host immune response. The effect of quorum sensing on cytotoxicity and the production of proinflammatory mediators was examined using the murine macrophage cell-line RAW264.7. Cytotoxicity was determined by measuring lactate dehydrogenase release in the culture medium. Extracellular products from luxS- and smcR-deficient mutants exhibited weak cytotoxic effects on RAW264.7 cells. The production of the proinflammatory cytokines TNFalpha, IL-1beta and IL-6 was measured with real-time PCR and ELISA, and production was measured with Griess reagents. Mutation of both luxS and smcR delayed the transcription of TNFalpha, IL-1beta and IL-6 genes. Also, levels of both TNFalpha and nitric oxide induced by luxS- and smcR-deficient mutants were significantly lower than those induced by parent strains. These results suggest that quorum sensing could be involved in the modulation of TNFalpha and nitric oxide produced from host cells by regulating virulence factors, and that V. vulnificus facilitates its host's mortality and bacterial survival by enhancing virulence on host cells.