The effects of an orally administered hot-water extract (HW) from cultured mycelia of Cordyceps sinensis on the activation of macrophages and the intestinal immune system were studied in mice. The general composition of HW was 83.9% carbohydrate, 11.8% protein, 1.9% lipid and 2.4% ash, and the carbohydrates were mainly composed of glucose, mannose, galactose and arabinose (molar ratio of 1.0:0.8:0.5:0.1). HW stimulated the activation (1.7-fold of the saline control) of macrophages and IL-6 production (1.5-fold) at 2.0 g/kg/day. Analyzing the culture supernatant of Peyer's patch cells from C3H/HeJ mice that had been fed with HW at 1.0 g/kg/day for 7 days indicated that the bone marrow cells had significantly proliferated (1.9-fold). In addition, the amounts of GM-CSF and IL-6 in the culture supernatant of Peyer's patch cells at the same dose were significantly increased (1.8-fold and 2.2-fold, respectively). These results indicate that an oral administration of HW may modulate IL-6 production by the activation of macrophages, and also enhance the secretion of hematopoietic growth factors such as GM-CSF and IL-6 from Peyer's patch cells. Since such cytokines as GM-CSF and IL-6 from Peyer's patch cells act on the systemic immune system, it can be assumed that orally administered HW modulated not only the local but also systemic immune system.