The inhibitory effects and their IC50 values of Tripterygium wilfordii (TW) on T and B lymphocytes were assayed using mice splenocytes induced to proliferate by mitogens ConA and LPS in vitro. The results indicated that 6 compounds (T4,T7,T8,T9,T10,L2) and component T1 had significant inhibitory effects on the proliferation of lymphocytes in response to ConA and LPS. The inhibitory potencies of these compounds were in the range of 10(-5)-10(-9) micrograms/ml for both T and B cells, while those of T1 were 3.5 and 4.2 micrograms/ml, respectively. The IC50 values, from low to high, were in the following order: T10,T4,T8,L2,T7,T9 and T1, The dose-effect curves for LPS were to the right of ConA curves, though both were nearly parallel. The relative order of the IC50 values determined from ConA and LPS curves were also coincident. Another compound, T11, showed no suppressive effect on the proliferation of T and B lymphocytes induced by either mitogen, even when its concentration reached 50 micrograms/ml. The results suggest that the suppressive effects of the componends on the proliferation of T and B lymphocytes in vitro were direct and non-selective.