The effects of radiotherapy on the immune competence of patients with mammary cancer was studied using enumeration of the various lymphocyte subpopulations as detected by monoclonal OKT antisera, lymphocyte proliferative responses to phytohemagglutinin, concanavalin A, purified protein derivative to tuberculin (PPD), and serum immunoglobulin levels. The tests were carried out in nine patients with newly diagnosed and operated mammary cancer before and after the radiation therapy, and 6 months later, and in ten patients whose mammary cancer had been diagnosed and treated at least 3 years earlier and who had remained tumor-free. The number of lymphocytes in the peripheral blood and the various T-cell subpopulations as well as the OKIa1 cells mostly representing B-cells decreased. The only population increasing were the OKM1-positive cells, mostly representing monocytes and null cells. As also described earlier, the responses to mitogens decreased because of the radiotherapy but recovered, except for the responses to PPD which remained low.