Immunity tests were conducted in 21 patients with cancer, before and two months after irradiation. T and B lymphocyte counts were normal before irradiation when compared to a control group. Lymphopenia was present after irradiation affecting mainly the T lymphocytes (P = 0.005), whereas changes in B lymphocytes were not significant. These results suggest that irradiation has an immunosuppressive effect which should be studied in greater detail to establish possible therapeutic applications.