The antirheumatic effects of triptolide, a purified component derived from a Chinese herb, Tripterygium wilfordii Hook f. (TWH), was examined. Peripheral blood mononuclear cells (PBMC), T cells, or human synovial fibroblasts isolated from healthy controls or rheumatoid arthritis (RA) patients were cultured in vitro in the absence or presence of triptolide. Estimated by ELISA, immunoglobulin synthesis in pokeweed mitogen or Staphylococcus aureus Cowan 1 strain stimulated PBMC was significantly impaired by triptolide in a concentration-dependent manner (1-10 nM). Similarly, proliferation of PBMC in response to phytohemagglutinin (PHA-M), interleukin-2, or phorbol 12-myristate 13-acetate (PMA)/ionomycin estimated by incorporation of [ 3 H]-thymidine was inhibited by triptolide. Cell viability was not affected at the immunosuppressive concentrations of triptolide. No abnormality of intracellular Ca 2+ flux as estimated by flow cytometry was detected in PHA-M-stimulated T cells by triptolide. Biosynthesis of cellular protein estimated by incorporation of [ 3 H]-leucine was significantly reduced in PMA/ionomycin stimulated PBMC by triptolide at concentrations above 7.5 nM. Proliferation of human synovial fibroblasts as estimated by crystal violet staining was significantly inhibited by triptolide at 30 nM. The present data demonstrate that triptolide is a potent immunosuppressant and has an antiproliferative effect on synovial fibroblast. The immunosuppressive activity of triptolide is not due to cytotoxicity, nor is it targeted at the initial membrane signal transduction process and the generation of second messengers. Inhibition of cellular protein synthesis by triptolide during lymphocyte activation may account for its inhibitory activity. The precise mechanism of action of triptolide needs to be defined in order to develop improved versions of the molecule for the potential treatment of RA.