3Beta-hydroxyolean-12-en-27-oic acid (1), a biologically active, pentacyclic triterpenoid isolated from the rhizomes of Astilbe chinensis, was found to be considerably more cytotoxic toward human colorectal carcinoma (COLO-205) and human cervical squamous carcinoma (HeLa) cells than its congener oleanolic acid (4). This suggests that the position of the COOH group significantly affects the cytotoxicity of oleanane-type pentacyclic triterpene carboxylic acids. To elucidate the underlying biological mechanism responsible for the cytotoxicity of 1, we investigated its growth-inhibitory effect on COLO-205 cells. Compound 1 induced a marked concentration- and time-dependent inhibition of cell proliferation, with the typical morphological characteristics of apoptosis, and under formation of DNA ladders in agarose-gel electrophoresis. Flow-cytometric analysis showed that the cell cycle of COLO-205 cells exposed to 1 was arrested in the G0/G1 phase. Also, 1 increased and decreased the expression of Bax and Bcl-2 proteins, respectively, and lowered the mitochondrial transmembrane potential (delta psi(m)). The peptidic caspase-3 inhibitor NH2-Asp-Glu-Val-Asp-CHO (at 1 microM) could increase the viability of COLO-205 cells previously treated with 1. These results indicate that 1 induces efficient cell apoptosis through down-regulating Bcl-2 expression, up-regulating Bax expression, lowering delta psi(m), and by activating the caspase-3 pathway.