[en] To elucidate the stimulative effect on cancer immunity system by low-dose irradiation, we studied the inhibition of spontaneous metastasis to lung after low-dose irradiation. Female WHT/Ht mice were inoculated the tumor cells and received whole-body X-ray irradiation with 15 cGy. The results obtained were as follows; (1) By an irradiation with 15 cGy, numbers of lung colony were decreased by 57% relative to the sham-irradiated controls. (2) By intraperitoneal injection of 200 μg of OK432, numbers of lung colony were decreased by 43-68% relative to controls. Furthermore, by combined treatment as OK432 i.p. with irradiation of 15 cGy, numbers of lung colony were decreased by 30%. (3) Mean diameter of the tumor were not decreased by these treatments. (author)