The effect of therapeutic irradiation upon the lymphocyte subpopulation has been investigated in patients with cervical uterine cancer. The influence of radiotherapy was studied on lymphocyte subpopulations analyzed by monoclonal antibodies such as anti-Leu 2a, anti-Leu 3a, anti-Leu 4, anti-Leu 12, and anti-Leu 11a by a FACS analyzer, and by the PHA (phytohemagglutinin) response in 29 patients with cervical uterine cancer. The absolute counts of the various lymphocyte subpopulations were seen to remarkably decrease during radiation therapy, however, their percentages remained unchanged. As for 17 patients with cervical uterine cancer who each received a radical operation, the Th/Ts (T helper T/suppressor) ratio was seen to slightly decrease during radiation therapy. In contrast, in the remaining 12 patients with cervical uterine cancer who each received radiotherapy alone, the Th/Ts ratio was found to be higher than the former cases. This suggests that this phenomenon might be caused by the presence or absence of a tumor.