Abstract.Opsonic activities of fish antibody and complement in the phagocytic responses of fish leucocytes have been investigated using salmonid peritoneal exudate cells (PEC) and viable cells ofAeromonas salmonicidaas antigenic particles. PEC were isolated from liquid paraffin‐injected coho salmon, masu salmon and rainbow trout. After incubation at 20°C for 2 h, phagocytosis by PEC was evaluated. Phagocytic activity was attributed to neutrophil‐like and macrophage‐like cells, though in the absence of immune serum activity was low. The addition of fish antibody (heal‐inactivated antiserum) slightly increased the activity. On the addition of normal serum as a complement source phagocytic activity was dramatically accelerated. This acceleration was interpreted as opsonization as a result of the classical pathway activation of fish complement by an antigen‐antibody reaction. The complement of mammals (goat, guinea pig and rabbit) did not possess opsonic activity in the presence of fis