Phagocytic activity and NBT reduction by blood granulocytes was evaluated in tench during the summer, when water temperature was high (30 degrees C). In vitro assays were performed at two temperatures, 30 degrees C, the temperature of the natural habitat in summer, and 22 degrees C, a commonly used temperature within the optimum range of warm-water fish. The results indicate that blood granulocytes from tench possess a lower capacity to ingest inert particles at 30 degrees C than at 22 degrees C, particularly during long periods of incubation (60 min). The lower capacity for ingesting inert particles at 30 degrees C is due to a decreased effectiveness of phagocytosis at this temperature, but not to a lower number of granulocytes with phagocytic capacity. The decline in inert particle ingestion capacity does not correspond to a lower production of superoxide anion at 30 degrees C, which is similar at both temperatures during phagocytosis, thus indicating a similar capacity for destruction of the antigen at 30 and 22 degrees C.