The apoptosis-inducing effect of c¢-anordrin (ANO) was investigated in this study. ANO 10-50 μM inhibited the growth of both human leukemia HL-60 and K562 cells by 19-52%. Electron microscopy showed that ANO-treated cells exhibited the drastic changes including cell shrinkage, chromatin condensation, nuclear fragmentation, typical of apoptosis. Gel electrophoresis of DNA extracted from both HL-60 and K562 cells treated with ANO revealed characteristic "ladder" pattern. ANO 50 IxM for 48 h caused approximately 50-70% apoptosis. Cycloheximide (CHX) and actinomycin D (Act D) did not prevent ANO-induced apoptosis in K562 cells, however, apoptosis of HL-60 cells in the presence of ANO was partially blocked by these two agents. Moreover, it was found that tamoxifen synergically potentiated and estradiol partially antagonized ANOinduced apoptosis in HL-60 cells. Results demonstrated that ANO could induce tumor cell apoptosis, which might contribute to its anticancer action.