An investigation of the immunologic responsiveness of Tursiops truncatus to Erysipelothrix insidiosa and Clostridium perfringens Type D antigens was reported. Serum protein changes were recorded by electrophoresis, and the induction time, degree and duration of the immune response were determined by immunofluorescence. Passive protection studies were conducted in mice using porpoise serum dilutions and an LD₅₀ of E. insidiosa and a TD₅₀ of the epsilon toxin of Cl. perfringens Type D. Statistically significant quantitative changes in beta-2 globulins were observed in T. truncatus following inoculation with either E. insidiosa bacterin or Cl. perfringens Type D bacterin-toxoid. Newly captured T. truncatus possessed circulating antibodies to E. insidiosa and Cl. perfringens Type D; these antibodies were demonstrated using indirect immunofluorescence. Serum from T. truncatus having a 1:1024 titer to E. insidiosa prevented the development of clinical signs of disease and bacteremia produced by an LD₅₀ of E. insidiosa in mice.