Tilapia was exposed by intraperitoneal injection to the environmental contaminants lindane, DMBA, and TCDD in separate experiments. Spleen and pronephros cellularities were reduced, usually in a dose-related manner, by each chemical agent. The functional capacity of phagocytic cells isolated from both fish spleen and pronephros was also evaluated as an indicator of chemical-induced immuno-toxicity. The ability of pronephros and spleen macrophages to phagocytize foreign particles (fluorescent microspheres) was not altered by exposure to lindane, DMBA, or TCDD at levels that did not produce overt toxicity. Phagocytosis was found to be reduced at doses of DMBA associated with reduced activity and increased pigmentation in fish. Similar results were obtained when the macrophage chemiluminescent response was evaluated, with no depression of superoxide radical (hydrogen peroxide) production occurring after exposure to lindane, DMBA, or TCDD. Depression of superoxide radical production occurred at toxic levels of DMBA. These data suggest that total immune cell counts in fish hematopoietic organs may represent more sensitive indicators of environmental immuno-toxicant exposure, than does phagocytic activity or chemiluminescent response of macrophages contained within these organs.