Orally administered SSG, a beta-1,3-glucan obtained from the culture filtrate of the fungus Sclerotinia sclerotiorum IFO 9395, was examined for effects on immune responses in mice. The proliferative responses of spleen cells from SSG-administered mice (40 or 80 mg/kg, daily for 5 or 10 consecutive days) to a T-cell mitogen, concanavalin A (Con A), or a B-cell mitogen, lipopolysaccharide (LPS), were higher than those from normal mice. Oral administration of SSG (80 mg/kg) to mice also enhanced the activities of both natural killer (NK) cells in spleen and the lysosomal enzyme of peritoneal macrophages. Furthermore, significant inhibition of tumor growth was observed in syngeneic tumor systems when SSG was administered directly after tumor implantation. The inhibiting effect required high doses of SSG (over 80 mg/kg). These results demonstrate that SSG can potentiate the immune response of mice following oral administration.