Rainbow trout (Salmo gairdneri) were exposed to the O-antigen ofYersinia ruckeriby various immunization regimens. The passive hemolytic plaque assay was used to show specific splenic plaque-forming cells (PFC) and passive hemagglutination demonstrated humoral antibody titers in fish injected with or immersed in the antigen preparations. Preceding antigen immersion with a 2-min immersion in either a 5.2 or a 2.6% NaCl solution did not affect the numbers of PFC or levels of antibody. Fish held for 2 min in ethylm-aminobenzoate methanesulfonate (MS-222®) before antigen immersion showed no significant difference from the control fish. Dosage studies showed that the minimal concentration of antigen for induction of PFC by immersion was 5.0 μg/mL.Key words: plaque-forming cells, immunization, rainbow trout immersion,Yersinia ruckeri