Immune responses to formalin-killed Aeromonas salmonicida cells and goldfish red blood cells (GFRBC) were assessed by assaying agglutinin titre and haemolysin titre, using a few different types of rainbow trout Salmo gairdneri (yearlings weighing 50-80g). These included steelhead trout, which is sea run rainbow trout, albino rainbow trout, obtained as a mutant, and unspotted rainbow trout (which is called “Horai masu”), a mutant lacking black-pigmented spots in the skin. The anti-A. salmonicida agglutinin reached the maximal titre after 2 weeks and still maintained a detectable titre level after 4 weeks. The anti-GFRBC haemolysin, however, formed a peak after 1 week and became undetectable after 3 weeks in antibody titration. Thus, a dissimilarity in antibody responses was produced by applying the two kinds of particulate antigens. In the comparative assessment of antibody responses of the trout, no significant difference between antibody assays could be traced, even though there were different types of rainbow trout among the immunized fish. The results appear to show that the types bear a close resemblance to each other in immunological potentiality, especially in antibody productivity.