Immunological and structural changes in the thymus and pronephros of Cyprinus carpio infected with the blood fluke, Sanguinicola inermis for 30 days, and exposed to 0.5 mg NH4+/1 for 48 and 168 h were investigated. Ultrastructural observations revealed cell disruption and highly vacuolated cytoplasm in the thymus. Of the cells that remained intact there was a significant increase in thrombocytes after 48 h exposure to the pollutant. In addition, there was a decrease in lymphocytes following exposure to ammonia at both time-periods studied. In contrast the pronephros of fish exposed to the pollutant underwent relatively mild changes in cellular architecture although ammonia and time of exposure had significant effects on the proportions of several leucocyte types. A significant decrease in neutrophils, thrombocytes and lymphocytes occurred in fish exposed to the pollutant for 168 h. Pronephric lymphocyte stimulation (cpm) by Con A and PWM increased in vitro, whereas the stimulation index was reduced in infected fish exposed to ammonia. Changes in the immune organs of S. inermis-infected carp treated with pollutant were both organ- and time-specific. The possible reasons for this are discussed and significance in relationship to parasitization assessed.