Triptolide is a naturally occurring diterpene triepoxide whose anti-inflammatory effects correlate with transcriptional inhibition of various cytokines. Despite its use in herbal medicine for thousands of years, the cellular target and mode of action of this drug are unknown. [3H]-triptolide was prepared and a filtration assay designed to measure binding to cells and cellular extracts. Triptolide bound specifically and irreversibly to a single, 90 kDa protein in nuclear extracts from stimulated and non-stimulated monocytic and epithelial cell lines. Thiol reactivity of one or more of the epoxides on triptolide was necessary for the covalent binding, since thiol oxidizing agents dithiodipyridine and diamide, and the thiol alkylating agent N-ethylmaleimide all reduced the binding of [3H]-triptolide to nuclear extract. Neither glutathione nor the pro-oxidant tert-butylhydroperoxide affected the binding of [3H]-triptolide to the nuclear protein, ruling out a general oxidant effect. The number of epoxide moieties correlated with the ability to compete with radiolabeled triptolide for binding to the nuclear extract and with the potency of inhibition of TNFalpha secretion from monocytes, IL-2 secretion from Jurkat cells, and with inhibition of RNA synthesis. The correlation between the structure-activity relationship and observed binding suggests that identification of the triptolide binding protein could provide insight into the cellular mode of action of this anti-inflammatory natural product.