The present study was undertaken to gain insights into the molecular mechanism of cell death (apoptosis) by guggulsterone, a constituent of Ayurvedic medicinal plant Commiphora mukul, using PC-3 human prostate cancer cells as a model. The viability of PC-3 cells, but not a normal prostate epithelial cell line (PrEC), was reduced significantly on treatment with guggulsterone in a concentration-dependent manner. Guggulsterone-mediated suppression of PC-3 cell proliferation was not due to perturbation in cell cycle progression but caused by apoptosis induction characterized by appearance of subdiploid cells and cytoplasmic histone-associated DNA fragmentation. Guggulsterone-induced apoptosis was associated with induction of multidomain proapoptotic Bcl-2 family members Bax and Bak. Interestingly, the expression of antiapoptotic proteins Bcl-2 and Bcl-xL was initially increased in guggulsterone-treated PC-3 cells but declined markedly following a 16- to 24-hour treatment with guggulsterone. Ectopic expression of Bcl-2 in PC-3 cells failed to confer significant protection against guggulsterone-induced cell death. On the other hand, SV40 immortalized mouse embryonic fibroblasts derived from Bax-Bak double knockout mice were significantly more resistant to guggulsterone-induced cell killing compared with wild-type cells. Guggulsterone treatment resulted in cleavage (activation) of caspase-9, caspase-8, and caspase-3, and guggulsterone-induced cell death was significantly attenuated in the presence of general caspase inhibitor as well as specific inhibitors of caspase-9 and caspase-8. In conclusion, the present study indicates that caspase-dependent apoptosis by guggulsterone is mediated in part by Bax and Bak.