Fresh channel catfish (Ictalurus punctatus) serum from unimmunized catfish exhibited 100% bactericidal activity against Salmonella paratyphi. Components responsible for bactericidal activity could be absorbed from the fresh catfish serum with S. paratyphi. The bactericidal system of the fresh catfish serum showed a need for magnesium rather than for calcium after EDTA treatment. The addition of salicylaldoxime or ammonium hydroxide to catfish serum indicated the alternate rather than the classical pathway of complement activation to be important in bactericidal activity against S. paratyphi. Bactericidal activity of catfish serum was labile when incubated at 47 degrees C for 30 min., stable for at least 4 mo. at -80 degrees C and could be absorbed with S. paratyphi at 25 degrees C. Very minimal bactericidal activity was present in the descending portion of the first 13.7S peak with most activity being found in the descending portion of the second 7.1S peak and throughout the entire 3.4S peak after Sephadex G-200 catfish serum fractionation.