Effects of synthetic cytidine-phosphate-guanosine (CpG) oligodeoxynucleotide (ODN) on respiratory burst activity of olive flounder (Paralichthys olivaceus) head-kidney phagocytes and on protection against lethal infection with Edwardsiella tarda were investigated. Phagocytes precultured with a CpG ODN showed significantly higher chemiluminescence (CL) responses than phagocytes precultured with guanosine-phosphate-cytidine (GpC) ODN or culture medium alone (control) at all concentrations. Supernatants produced from leucocytes, which were pulsed with CpG ODN, induced significantly higher respiratory burst activity than supernatants produced by GpC ODN or culture medium alone. In an in vivo experiment, respiratory burst activities of the head kidney phagocytes in the groups injected either 0.25 or 0.5 microg fish(-1) of CpG ODN were significantly higher than those in the groups injected with GpC ODN or HBSS (control) at 3, 5 and 7 d after injection. The groups of fish injected with 0.25 or 0.5 microg of CpG ODN showed higher survival rates (83.3%) than groups treated with GpC ODN (33.3%) and a control group (8.3%) after challenge with E. tarda. The present in vitro and in vivo experiments have demonstrated the ability of synthetic CpG ODN to increase phagocyte respiratory burst activity and disease resistance in olive flounder.