The influence of stress upon the endocrine system and its relationship to the immune system have been studied. The simulation of chronic cold stress in rats caused an increase in ACTH, corticosterone, and T3 and T4 levels. The different hormonal background in stressed animals affected the characteristics and the intensity of the immune response, which was traced in dynamics and through the action of 10(9) ovine erythrocytes. A strong suppression of B-lymphocytes and of PFC count was observed. Increased Fc gamma-RFC-suppressing activity was also determined. The PFC suppression changed simultaneously with the rate and dynamics of Fc gamma-RFC-suppressing activity in animals under stress. In control rats (immunized with ovine erythrocytes only), the antigen caused insignificant changes in corticosterone level, which was not typical for stress and was not accompanied by suppressing activity in the immune response.