This study was aimed to evaluate the effect of lentinan on the immune function of splenic lymphocytes in rotary cell culture system (RCCS) microgravity environment. The splenic lymphocytes from mice were separated and cultured in the normal gravity and the microgravity environments. The cells were treated with lentinan solution (0, 10, 20 and 40 µg/ml). After incubated with lentinan for indicated times (24, 48 and 72 h), the cell proliferation, secretion of cytokine and the expression of cell surface markers were detected by MTT method, ELISA and flow cytometry respectively. The results indicated that lentinan of above mentioned concentrations did not obviously promote the lymphocyte proliferation, but increased the secretion of IL-2 and IFN-γ and enhanced the expression of lymphocyte surface markers CD4 and CD8 in microgravity environment. It is concluded that lentinan has the ability to enhance the lymphocyte immune function in microgravity environment.