Over one quarter of all cancer patients receive mediastinal irradiation as part of their primary cancer treatment and another quarter will receive it secondary to palliative X-ray treatment for metastatic disease in the mediastinum or spine. Immunosuppression, presumably caused by thymic irradiation, is an undesirable side effect of this treatment but little is known with respect to the functional significance and duration of the deficit, particularly with respect to the potentially beneficial effects of anti-tumor immunity mediated by thymus-derived lymphocytes. This investigation is designed to measure these deficits quantitatively and prospectively in a group of patients with breast cancer already randomly assigned between control and irradiated groups as part of the National Surgical Adjuvant Breast Project. A second set of controls, patients with cervical carcinoma, will receive similar radiation doses to lymph nodes not to the thymus. The immune competence of each patient will be assessed by a panel of skin tests before and after radiation therapy, and periodically thereafter in selected patients. Thymus-derived lymphocytes will be enumerated using rosette and membrane immunofluorescence tests and their functional capacities assessed by measuring stimulation of DNA synthesis by mitogens or antigens and production of two effectors of cell-mediated immunity, migration inhibitory factor and lymphotoxin. Patients will be followed to determine the course of the disease. The study is designed to determine whether any enduring deficit in cellular immunity occurs from thymic irradiation, which specific subpopulations are affected, and the relationship of immunosuppression to prognosis.