Combination effect of lentinan with X-ray irradiation on the metastatic mouse tumors, L1210, KLN205 and Lewis lung carcinoma were studied. Combination use of lentinan with X-ray therapy prolonged the life of BDF1 mice bearing L1210 leukemia in the suitable combination conditions. Combination effects of lentinan with X-ray therapy were also observed on the suppression of the growth of KLN205 squamous cell carcinoma and on the suppression of the metastasis of Lewis lung carcinoma. Especially, in the case that lentinan was administered before or after X-ray local irradiation in the pulmonary metastasis system of Lewis lung carcinoma, a marked suppression of pulmonary metastasis was observed and 2 to 4 mice among 8 tested mice were tumor free.