The Authors have studied the radiosensitivity of lymphocyte cultures of patients affected by systemic lupus erythematosus in the active phase, and in the remissive phase after treatment with cyclosporine and fluocortolone, compared to a group of normal subjects. Total lymphocyte cultures and cultures of T, B, NK lymphocytes, T helper/inducer and T suppressor cytotoxic obtained with specific monoclonal antibodies were used and then irradiated with Co-60 gamma photons at scale doses between 0 and 10 Gy. Damage due to irradiation was evaluated using 3H-TdR. Patients in the clinically active phase of the disease showed an increased lymphocytic sensibility of the total lymphocytic population as well as of the various populations and T subpopulations studied. Radiosensitivity tended to become normal in patients in the remissive phase of the disease. The Authors conclude that a preliminary study of the lymphocytic sensitivity should be carried out in all patients treated with radiotherapy with autoimmune pathologies in order to avoid damages due to irradiation. Moreover, the Authors retain that a test for lymphocytic sensitivity to radiation could apply with regard to the evaluation of the clinical remissive conditions of patients affected by systemic lupus erithematosus.