Spatholobi Caulis has been used in Oriental medicine to treat cancer and blood stasis. In this study, the methylene chloride fraction of Spatholobi Caulis (MCSC) was examined to determine if it possesses anti-cancer activity via its apoptosis-inducing activity. MCSC exhibited a strong cytotoxic effect against human monocyte leukemia U937 cells (IC(50)=15.1 microg/ml). A TUNEL assay showed that the MCSC caused a characteristic ladder pattern of discontinuous DNA fragments and apoptotic bodies. Flow cytometric analysis confirmed that MCSC significantly increases the number of apoptotic cells stained by annexin V(+)/PI(-) cells. Western blotting revealed that MCSC activated caspase-3 expression and cleaved poly (ADP-ribose) polymerase (PARP) in a concentration-dependent manner. An enzyme-linked immunosorbent assay (ELISA) demonstrated that MCSC significantly activated the caspase-3 activity compared with the untreated control by. Taken together, these results suggest that MCSC can induce apoptosis in U937cells via the caspase dependent pathway.