Sixteen patients with early stage carcinoma of the uterine cervix treated with primary radical hysterectomy and pelvic lymphadenectomy were compared with 17 patients who four to six weeks before the operation received intracavitary treatment with radium. The calculated radiation dose to the pelvic wall was approximately 10 Gy. The distribution of lymphoid cells in blood and pelvic lymph nodes was studied by an indirect immunofluorescence technique using monoclonal antibodies. The radium treated group showed a significant reduction of circulating OKT4+ (T helper) and OKT8+ (T suppressor/cytotoxic) lymphocytes. The number of Leu7+ (natural killer) cells and 1D5+ cells (monocytes) was not changed, but the ratio between monocytes and T cells was increased after radium therapy. In cell suspensions obtained from the pelvic lymph nodes, the radium treatment induced a significant reduction of the OKT4+ cell fraction. It is concluded that this low dose rate regimen of intracavitary treatment induces changes in the immune system which are of the same type as those seen after external field irradiation.