The pathogenicity of four strains of Erysipelothrix rhusiopathiae, isolated from marine mammals, were evaluated in mice.  LD50 values ranged from 1 to 55 depending on the source of the isolates. The strains also exhibited differences in agglutinating antibodies in sera of E. rhusiopathiae-vaccinated dolphins (Tursiops truncatus) based on tube agglutination titer assays. Agglutinating antibodies from the serum of an immunized dolphin were separated by ammonium sulfate precipitation and DEAE Sephadex column chromatography.  The fractions yielding the highest anti-E. rhusiopathiae activity, using strain SW-42 as antigen, eluted with IgA and IgM.  The lack of participation by serum IgG in E. rhusiopathiae neutralization was further indicated when purified preparation of IgG were substituted for whole serum in the agglutination assay.  In addition, very little radioactivity was associated with the pellet formed in agglutination tubes when ¹²⁵I-labeled Protein A was added either during or after reaction of E. rhusiopathiae antigens with whole serum of immunized dolphins.  The results suggest that long-term protection of dolphins by the Fort Dodge vaccine against E. rhusiopathiae is mediated by either IgA and/or IgM with little or no involvement of IgG.  These findings are useful in the development of an ELISA or RIA for evaluation of serum antibody levels against this bacterium in dolphins.