The in vitro effects of the Chinese Herba Epimediia glycoside icariine (ICA), a chinese herbal extract, on human immune responses were investigated. ICA induced a weak and delayed proliferation of peripheral blood mononuclear cells from healthy donors when compared to phytohemagglutinin. Both T- (TCR alpha beta+) and B cells were the ICA-responding cells. Within T-cells, the relative proportion of CD4-8+ cells increased but that of CD4+8- cells decreased. ICA in certain concentrations could increase lymphokine-activated killer cell (LAK) activity (0.1 to 1.0 microgram/ml) in both tumor patients and healthy donors, and natural killer cell (NK) activity (1.0 to 5.0 micrograms/ml) in tumor patients. Moreover, ICA stimulated the production of tumor necrosis factor-alpha in monocytes from healthy donors. These findings provide evidence that ICA could be applied to adoptive immunotherapy. Generation of LAK cells in presence of an appropriate dose of ICA might be superior to interleukin-2 alone.