The alkaloid Sinomenine extracted from the medicinal plant Sinomenium acutum is used in China for the treatment of various rheumatic diseases. It has immunomodulatory properties in a cardiac allograft transplantation model. Its antiproliferative effect on human mononuclear cells in combination with different immunosuppressive drugs was further analysed in vitro. Sinomenine dose-dependently attenuated thymidine incorporation, interleukin-2 synthesis, and cell cycle progression of activated T-lymphocytes. Cell proliferation was synergistically decreased by addition of Sinomenine together with sub-optimal concentrations of the established immunosuppressive drugs tacrolimus or mycophenolic acid, respectively.