The mechanisms of the cellular immune response involved in the protection of fish against infection by the pathogenic bacterium Vibrio anguillarum are largely unknown. In the present study, sea bass specimens were injected with live or formalin-killed V. anguillarum and the respiratory burst of leukocytes was measured. The infection of fish resulted in a strong inhibition of the respiratory burst, in contrast with the slight increase in respiratory burst of leukocytes from fish injected with dead bacteria. In addition, we observed a concomitant down-regulation of p22(phox) and p40(phox), two components of the NADPH oxidase, in the leukocytes from infected fish. To investigate whether these differences may be the result of a dysregulation of cytokines expression in infected fish, we cloned several sea bass cytokines, including interleukin-6 (IL-6), IL-8 and three CC chemokines, and performed a detailed expression study with these and other cytokines. Surprisingly, cytokine expression was fairly similar in leukocytes from both live and formalin-killed V. anguillarum-challenged fish, the response being even higher and longer lasting in infected fish. Furthermore, the expression of two key apoptotic caspases, caspase-3 and -9, was down-regulated in leukocytes from infected fish, but remained unaltered in fish injected with formalin-killed bacteria. These results suggest that the virulence mechanisms of V. anguillarum in sea bass involve the inhibition of leukocyte respiratory burst and apoptosis, and thereby providing a safe haven for growth.