Circadian variations in the sensitivity of blood lymphocyte succinate dehydrogenase to thymalin and hydrocortisone are shown. The maximal sensitivity to thymalin is observed in the morning, and to hydrocortisone in the evening. The range of fluctuations in this parameter is wider in patients with immunodeficiency than in donors. Moreover, the direction of the lymphocyte metabolic reaction to the studied agents was found to be disturbed in the patients. The findings may be useful for designing chronotherapeutic schemes with the use of hormonal immunomodulating agents.