Tripterygium wilfordii Hook F (TWH) is a vine‐like plant that grows in a wide area of south China. An alcohol extract of this plant known as T2 has been suggested to be effective in the treatment of rheumatoid arthritis (RA). To examine the mechanism by which this herbal remedy might be effective in RA, the capacity of T2 to alter human immune responsiveness in vitro was investigated. Human peripheral blood mononuclear cells were obtained from normal adults and separated into purified populations of monocytes, T cells, and B cells. T2 at 0.1–1 μg/ml inhibited antigen‐ and mitogen‐stimulated proliferation of T cells and B cells, interleukin‐2 (IL‐2) production by T cells, and immunoglobulin production by B cells. T2 did not affect IL‐2 receptor expression by T cells, IL‐1 production by monocytes, or the capacity of monocytes to present antigen. Inhibition could not be accounted for by nonspecific toxicity. These results support the conclusion that T2 exerts a powerful suppressive effect on human immune responses. This action might account for its therapeutic effectiveness in RA.