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The glucan that was produced by glucosyltransferases (GTFs) from Streptococcus mutanswas examined for its stimulating functions toward murine peritoneal macrophages. Soluble glucan was obtained by the reaction with cell-free crude GTFs and sucrose, followed by ethanol precipitation, dispersion in water and re-precipitation by ethanol. Soluble glucan, those average molecular weight was about 3×105, was composed of mixture of α-1,6 and α-1,3 linkages in a 3:1 ratio. When 30 and 60 μg/ml of the glucan was incubated with peritoneal macrophages, the lysosomal phosphatase activity was increased in a dose-dependant manner, indicating that soluble glucan may activate macrophages. To examine its effects on the various functions of macrophages, soluble glucan was orally administered daily at a level of 100 mg/kg of body weight to C57BL/6 mice. Significant stimulation of the production of H2O2by the macrophages was observed without any increase in NO production. The production of tumor necrosis factor-α (TNF-α) by the macrophages was also stimulated from 538.73–555.06 pg/ml to 585.73–596.40 pg/ml during 15 days of oral administration of soluble glucan. The cytotoxicity of peritoneal macrophages against B16 tumor cells was significantly enhanced by 25–38% during 15 days of oral administration. These results may indicate that soluble glucan stimulates the immune functions of macrophages.