The extract of Tripterygium wilfordii Hook F. (TwHF), which showed anti-inflammatory and immunosuppressive activities in human clinical trials for rheumatoid arthritis, was subjected to the activity-guided fractionation and spectroscopic characterization of bioactives. A tetrahydrofuran lignan, tripterygiol (1), and eight known compounds, all capable of suppressing pro-inflammatory gene expression were identified. Most of the pharmacological activity of the extract can be attributed to triptolide, its most abundant and active component, with some contribution from tripdiolide.