The effects of polypeptide thymic and bone marrow factors on the cyclase system of rat thymus and spleen lymphocytes were studied. It was shown that the induction of maturation of rat T-lymphocyte precursors into immunocompetent T-cells under effects of the thymic factor is accompanied by the activation of the adenylate cyclase system and the elevation of the cAMP/cGMP ratio. The observed increase in the cGMP level in splenic lymphocytes suggests the presence of cAMP- and cGMP-dependent steps of in the reaction of T-lymphocyte maturation. The bone marrow factor causes an elevation of the cAMP level only in splenic lymphocytes, which points to differences in the lymphocyte subpopulations that are sensitive to thymus and bone marrow factors. Impairments in T-lymphocyte maturation in patients with immunodeficiencies are concomitant with shifts in the isoenzyme spectrum of lactate dehydrogenase in peripheral blood lymphocytes as well as with changes in the sensitivity of the cAMP system to the thymic factor and isoproterenol. These disturbances are relieved by administration of the thymic factor. The roles of the cyclase system and polypeptide thymic and bone marrow factors in the molecular mechanisms of immunocompetent cell maturation are discussed.