Alterations of immune parameters were investigated in fish exposed to non-overtly toxic levels of three different environmentally relevant chemicals, 7,12-dimethylbenzanthracene (DMBA), hexachlorocyclohexane (lindane), and 2,3,7,8- tetrachlorodibenzo-p -dioxin (TCDD). Each chemical agent was administered to tilapia in separate experiments by intraperitoneal injection for five consecutive days. Following the final dose, total cellularity and histology of the spleen and pronephros were assessed, as were activity of phagocytic celis contained in these hematopoietic organs. <p>Using chemical doses which produced no clinical toxicity, tilapia exposed to each chemical agent displayed a significant reduction in total cell number of both spleen and pronephros, in most cases in a dose-related manner. Consistent with this observation, splenic and pronephric hypocellularity was confirmed upon histological examination of chemical-treated fish. However, neither superoxide radical production or phagocytosis of splenic or pronephric macrophages was inhibited in either DMBA, lindane, or TCDD exposed fish. Results of this study indicate that depressed total cell number in fish hematopoietic organs may be a more sensitive indicator of exposure to these environmental contaminants than is the activity of macrophages contained within these organs. / Master of Science