The exopolysaccharide (EPS) is a polysaccharide from cultivated Cordyceps sinensis, which possesses immunomodulatory and antitumor effects, was purified by DEAE-32 cellulose and Sephadex G-200 gel. The preliminary characters of EPS were analyzed by IR and GC, and the molecular weight was estimated by gel filtration. The effect of EPS on proliferation ability of lymphocytes from ICR mice was assayed by MTT method. The mRNA and protein expression levels of several cytokines in spleen and thymus cells were detected by RT-PCR and ELISA. The results showed that EPS consists of mannose, glucose, and galactose in a ratio of 23:1:2.6. Its molecular weight is about 1.04x10(5). EPS elevated proliferation ability of spleen lymphocytes only at 100 mu g/ml after 48 h treatment. Tumor necrosis factor alpha (TNF-alpha), interferon-alpha (IFN-gamma), and interleukin-2 (IL-2) mRNA levels in splenocytes and thymocytes were increased after EPS treatment for 2, 4, 8, or 20 h. EPS also significantly elevated splenic TNF-alpha and IFN-gamma protein expressions at 100 mu g/ml and increased thymic TNF-alpha and IFN-gamma protein levels at 50 and 100 mu g/ml. These data indicated that EPS may stimulate cytokine expressions of immunocytes.