The golden shiner, Notemigonus crysoleueas, developed antibody titer of 1,280 in 59 days from multiple, intraperitoneal injections of a formalin-killed bacterin of Acromonas liquefaciens. After four immunizing injections there was a significant increase in the time of survival of vaccinated fish over nonvaccinated fish challenged with viable A. liquefaciens. Analysis of golden shiner sera by paper electrophoresis produced a pattern of six protein components. Antibody activity was associated with a component which had a slow anodic electrophoretic mobility and solubility of gamma globulin. During the course of vaccination, increments in the serum protein component and in antibody titer coincided. Also, the amount of the protein component of vaccinated fish was decreased by specific adsorption of antibody by homologous antigen. Thus, a fish immunoglobulin was identified which had certain physical and chemical properties correspondng to classical definitions of gamma globulin.