Immunosuppressive drugs have been developed from natural products such as soil and fungi, which are also the sources of some commonly used herbal products. However, the effect of herbal products on immune response has not been investigated. Because these products can affect the host immune system they can induce either rejection or tolerance after a transplant procedure. To investigate the effects of ten commonly used herbal products on transplant-related immune function we performed in vitro lymphocyte proliferation tests using phytohemagglutinin, mixed lymphocyte culture (MLC) assay, and interleukin (IL)-2 and IL-10 production from MLC. Dong quai, ginseng, and milk thistle had nonspecific immunostimulatory effects on lymphocyte proliferation, whereas ginger and green tea had immunosuppressive effects. Dong quai and milk thistle increased alloresponsiveness in MLC, whereas ginger and tea decreased these responses. The immunostimulatory effects of dong quai and milk thistle were consistently seen in both cell-mediated immune response and nonspecific lymphoproliferation, whereas that of ginseng was not. The immunosuppressive effect of green tea and ginger were mediated through a decrease in IL-2 production, but the immunostimulatory effects of dong quai and milk thistle were not. We conclude that green tea, dong quai, ginseng, milk thistle, and ginger have effects on in vitro immune assays that may be relevant in transplantation in humans.