Abstract <p>A study of some parameters of nonspecific humoral and cellular immunity has been carried out in sexually mature bream individuals living in the reservoirs of the Middle Volga (Kuybyshev, Cheboksary, and Gorky). The level of antimicrobial properties and C-reactive protein of blood serum and nonspecific immune complexes in blood serum and tissues of kidney, spleen, and liver has been studied, and the share of immunodeficient individuals and phagocytic activity of neutrophils has been determined. The highest indices of bacteriostatic activity of serum, average cytochemical coefficient of neutrophils, and positive level of C-reactive protein of peripheral blood in most of the individuals are noted in the Kuybyshev and Cheboksary reservoirs. The content of nonspecific immune complexes in bream of the Cheboksary reservoir exceed similar data of fish from other reservoirs. No immunodeficient individuals are recorded in the Gorky Reservoir. The differences are probably related to the impact of different pathogenic factors on the organism of bream.</p>