A formalin-lulled cell (FKC) preparation of Edwardslella tarda was injected once intramuscularly lnto eels to study changes in agglutinating antibody titers and serum proteins Blood samples were taken at 1, 2, 3, 5, 8, and 1 2 wk after immunization. Agglutinating antibody titers were determined, and serum protein changes were investigated by electrophoresis and filtration on Sephadex G-200. At 25 "C, eels produced agglutinating antibodies against E. tarda FKC within 1 wk, with the maximum response occurring at 3 wk after immunization. There were no significant differences between immunized and non-immunized eel sera using cellulose acetate and polyacrylamide electrophoresis or immunoelectrophoresis. By gel filtration of the serum, 4 fractions were obtained but only the first showed specific agglutinating activity against E. tarda FKC. This activity was lost after treatment with 2-mercaptoethanol. The results suggest that in eels, the substance responsible for specific agglutinating activity in the serum corresponds in certain properties to human IgM.