The proportion of T and B lymphocytes in the peripheral blood was determined in patients with either mammary cancer or with various pelvic malignancies. In cancer patients studied prior to irradiation the level of cells forming either E‐rosettes or EAC'‐rosettes was similar to that found among healthy controls. Radiation therapy resulted in a striking lymphopenia. The level of cells with T‐cell markers was diminished to a greater extent than the level of cells with B‐cell markers. The relative proportion of T‐cells forming high affinity E‐rosettes was not reduced following radiation, so that it can be concluded that radiation affects predominantly the subpopulation of T‐cells which do not form high affinity E‐rosettes. Irradiation of the pelvic area resulted in a more rapid reduction of the level of T lymphocytes than irradiation of the mediastinum, although the final relative proportions of the cells were similar in both groups of patients. The results of the present study suggest that the reduction of the level of T lymphocytes following irradiation results from its effect on the lymphocytes in the major blood vessels, and that radiation of the thymus is not a prerequisite for this phenomenon.