Physiological parameters in fish provide grounds for conclusions as to the physiological status of the organism. The status is directly related to inner and outer factors, biotic and abiotic influences which act on the organism. Present studies aimed at defining the set of immunological and haematological indices in breams, which form fish populations in D.bie Lake and Szczecin Bay, whose ecosystems differ in water cleanness. Immunological studies in breams involved determining selective defensive functions of neutrophilic granulocytes, expressed by the index of phagocytosis of a standard bacterial strain (Ipg), the percentage of phagocytes which ingested the bacteria (%gp), NBT index, the amount of formazan and the index of myeloperoxidase activity (WA MPO). Haematological studies included determination of absolute leukocyte number and the differential leukocyte pattern. Bacteriological and physicochemical studies were also performed on water samples of reservoir sites from which the fish originated. Results of examination of water samples from D.bie Lake and Szczecin Bay demonstrated a significant level of water pollution in the ecosystems. A definitely higher level of water pollution, particularly in respect to bacterial contamination, was observed in D.bie Lake. Comparison of immunological and haematological results demonstrated significant differences between the two studied bream populations, i.e. those originating from D.bie Lake and the other, originating from Szczecin Bay. Such differences were disclosed both in immunological and in haematological indices.