The combined effect of PSK and radiation therapy has been studied in 34 patients with squamous cell carcinoma of the uterine cervix by noting the tumor cell kinetics change following irradiation. The DNA content of Pararosanillin-Feulgen stained tumor cells was measured by cytofluorometry to elucidate tumor cell kinetics. Twenty-one cases (PSK group) were administrated 3g. or 6g. of PSK daily until at least 30Gy. Thirteen cases (control group) were treated with external cobalt-60 irradiation without PSK. In radiosensitive cases (n = 29; PSK group: 19, control group: 10), the PSK group had a smaller Over 4C Cell population in the DNA histogram after 14Gy than the control group, and the population at 20 Gy in the PSK group was significantly smaller (p less than 0.05). At 20Gy, the PSK group showed better histopathological response than the control group according to the Ooboshi-Shimosato classification, and the PSK group showed a smaller giant cell formation and more colliquative necrosis. These findings indicated that Over 4C cell (giant cell) formation due to endomitosis was decreased because of the increase in lethal damage to tumor cells following PSK administration with irradiation. In radioresistant cases (n = 5) also, there seemed to be an advantage.