The potential role in pathogenicity of extracellular products (ECP) of the fish pathogen Aeromonas salmonicida was investigated. Evidence is presented that determinants of host damage are produced extracellularly by A. salmonicida. In vitro, the bacterium produced a number of extracellular enzymic and ctyolytic activities, including proteases and an heterogeneous haemolysin which was active against rainbow trout erythrocytes. Solubilization of trout erythrocytes in vitro resulted from the cooperative effects of the haemolysin and a caseinolytic protease. The caseinolytic protease was implicated in host damage; the in vivo effects of the caseinase may have been potentiated by haemolysin. The production of extracellular proteolytic and haemolytic activities by a number of isolates of A. salmonicida was studied; some potentially important differences between isolates were found. Evidence is presented that rainbow trout possess humoral mechanisms of resisting the toxic products of A. salmonicida. Proteolytic and haemolytic activities of ECP were inhibited, and the in vivo toxicity of ECP was neutralized, by normal trout serum. In vitro, factors in normal trout serum appearaed to form soluble complexes, as well as insoluble precipitates, with component(s) of ECP. In vitro, ECP caused a reduction in circulating levels of an alpha-2 macroglobulin analogue in rainbow trout. As putative determinants of pathogenicity, extracellular factors of A. salmonicida are potentially important in conferring protective immunity against furunculosis. The humoral immune response of rainbow trout and rabbits to components of ECP was investigated. At least 15 components of ECP, including the caseinase and haemolysin, were immunogenic in the rabbit. In the trout, antibodies to only 4 components of ECP were detected and no antibody to the caseinase or haemolysin was found. It is suggested that the effectiveness of furunculosis vaccines might be improved if the immunogenicity in trout of certain ECP components could be enhanced.