Triptolide (TL) is a major active component extracted from Chinese traditional herb Tripteryginm wil-fordii Hook. In this article, the effects of TL on mixed lymphocyte culture (MLC),suppressor T cell(Ts) activity, delayed type hypersensitivity (DTH) reaction, IL - 2 secretion activity and Th/Ts ratio were evaluated, In vitro TL 0. 5~10 ?g ? L-1suppressed one-way MLC,Lymphocytes induced in first MLC with TL of 5,10 ?g ? L-1 suppressed the second MLC after irradiation with 3000 rad 60Co source. This suggests that TL may induce Ts cell-s. In vivo, DTH reaction sensitized to dinitrofluo-robenzene (DNFB) monitored by the increase in weight of the ears challenged with antigen was suppressed by TL at doses of 0. 12 to 0. 50 mg ? kg-1(ip,qd?5) and the IL -2 activity secreted by spleen cells of these mice was inhibited at doses of 0. 25 and 0. 5 mg ? kg-1. TL 0. 25, 0. 5 mg ? kg-1(ip,qd?8) also had prominent suppres-sive effects on enhanced DTH reaction which was induced by cyclophosphamide(250 mg ? kg-1,ip). Th/Ts ratio of mouse thymus cells were reduced after given 0. 25 mg ? kg-1 of TL for 5 consecutive days. The above-mentioned data suggest that TL has suppressive effects on cellular immunity function and the suppressive mechanism may be related to the suppression of Th cells and IL-2 secretion activity and the induction of Ts cells.