Aim: To investigate the effects of triptolide on proliferation and apoptosis as well as on the expression of RIZ1 in the human multiple myeloma cell line U266 in vitro. Methods: The effect of triptolide on the growth of U266 cells was studied by MTT assay. Apoptosis was detected by Hoechst 33258 staining and Annexin V/PI double-labeled flow cytometry, and caspase-3 mRNA was measured by RT-PCR. Western blotting, flow cytometry and RT-PCR were used to assess the expression of RIZ1, and the location and expression of H3K9me1 were detected by confocal microscopy and Western blotting. Results: Triptolide significantly inhibited the proliferation of U266 cells in a time- and concentration-dependent manner (the IC50 value for a 24-h exposure was 157.19±0.38 nmol/L). Triptolide induced typical apoptotic morphological changes. Triptolide 40, 80, and 160 nmol/L treatment induced significant caspase-3-dependent apoptosis compared with control group (10.5%±1.23%, 37.9%±2.45%, and 40.5%±2.30% vs 3.8%±1.98%, P<0.05). Compared with peripheral blood monocular cells (PBMC) from healthy donors, the protein expression of RIZ1 in U266 cells was relatively low, but the mRNA and protein expression of RIZ1 were strikingly increased by triptolide in a concentration-dependent manner. Triptolide increased the protein expression of RIZ1 and RIZ1 methylates histone H3 lysine 9 in U266 cells. Conclusion: Triptolide increased the protein expression of RIZ1, inhibited the proliferation, and induced caspase-dependent apoptosis in U266 cells. © 2010 CPS and SIMM. All rights reserved.