Intraperitoneal injection of beta-glucan was shown to greatly delay mortality in mice exposed to whole-body X-ray radiation and tumor growth in tumor-bearing mice. Since the leukocyte and lymphocyte numbers were increased by a single dose of beta-glucan, the radioprotective effect of beta-glucan is probably mediated, at least in part, by a hemopoietic action in irradiated mice. In addition, both natural killer (NK) and lymphokine-activated killer (LAK) activities were significantly increased by repeated doses of beta-glucan. Augmented immunological activity as seen in increased NK and LAK activity by beta-glucan seems to play a role in preventing secondary infections associated with irradiation, and probably contributes to the attenuated tumor growth in tumor-bearing mice through enhanced anti-tumor immunity. These results suggest that beta-glucan may be a promising adjunct treatment for cancer patients receiving radiotherapy.