Adenyl cyclase activity was determined in suspensions of rat lymphocytes from animals with postadjuvant polyarthritis and from healthy animals. Adrenaline was used as activator of the enzyme. Adenyl cyclase activity was determined by the method based on synthesis of 3H-cAMP from 3H-ATP in intact cells. The influence of imidazole on blastic transformation of lymphocytes sensitized in the process of postadjuvant polyarthritis and subjected to the action of antigen, i.e. Old Tuberculin, was also studied. The response of sensitized lymphocytes to antigen in the presence of a nonspecific stimulus, adrenaline, was also investigated. In addition, the action of adrenaline on incorporation of 3H-thymidine in cultures of nonsensitized lymphocytes was observed. Adenyl cyclase activity in suspensions of lymphocytes from animals with postadjuvant polyarthritis was higher than in suspensions of lymphocytes from healthy animals. Following administration of adrenaline as activator of the enzyme, lymphocytes from healthy animals showed higher activity of the enzyme than lymphocytes from the sick rats. Imidazole inhibited the response of lymphocytes sensitized to antigen. Adrenaline had no influence on blastic transformation of lymphocytes sensitized to antigen. Adrenaline slightly inhibited incorporation of H-thymidine in cultures of sensitized lymphocytes in absence of antigen in cultures of nonsensitized lymphocytes.