Cordyceps sinensis (Chinese caterpillar fungus) is a fungus parasitic on the larvae of Lepidoptera and has been considered to be a precious tonic food and herbal medicine since ancient times in China. Recently, some fungal strains have been isolated from the fruiting bodies of wild C. sinensis, and some of them have been reported to show the same properties as the natural product. In the present study, an EPSF (exopolysaccharide fraction) was prepared from cultivated C. sinensis and its effects on B16 melanoma-bearing mice were investigated. Three doses of EPSF were intraperitoneally administered every 2 days after the day of tumour-cell injection. The experiment was terminated on day 28. Phagocytosis of peritoneal macrophages and proliferation of spleen and thymus lymphocytes were assayed. The tumour metastatic foci on the lung and liver surface were checked. The expression of oncoprotein Bcl-2 in livers and lungs was assayed by a immunohistochemical method. The results showed that EPSF significantly enhanced the Neutral Red uptake capacity of peritoneal macrophages (60 mg/kg, P<0.01; 120 mg/kg, P<0.001) and spleen lymphocyte proliferation (60 mg/kg, P<0.05; 120 mg/kg, P<0.001) in B16-bearing mouse. The metastasis of B16 melanoma cells to lungs (120 mg/kg) and livers (30, 60 and 120 mg/kg) was significantly inhibited by EPSF. Moreover, EPSF decreased the levels of Bcl-2 in the lungs (120 mg/kg) and livers (30, 60 and 120 mg/kg). These results suggest that EPSF has immunomodulatory function and antitumour activity.