SUMMARY A total of 360 Nile tilapia (O. niloticus) were used to compare the efficacy of formaline-killed vaccine (FKV, an insoluble antigen) and crude lipopolysaccha ride (LPS, a soluble antigen) against infection with Edwardsiella tarda. O. niloticus vaccinated either with FKV or crude LPS by immersion (DP) method were protected against infection with E. tarda although the antibody titers in the sera of both groups were similar to those of the control throughout the experimen
tal period. This protection of immunized fish was correspondence to the high level of secreted antibody in the body surface mucus. Meanwhile, the survival ra tio of O. niloticus vaccinated with crude LPS and FKV were stayed at 100 and 65% after 4 weeks from vaccination where the secreated antibody titers were 9 and 4 respectivelly. The survival ratio of unvaccinated fish was 35% after each of the 4 challenges where the natural secreted antibody titer was 1