Se-methylselenocysteine (Se-MSC) has been shown to possess potent chemopreventive and anti-tumor properties. However, its exact mechanism of action is still not well understood. The present study investigated the mechanism of Se-MSC on the induction of apoptosis using U937 human leukemia cells. Se-MSC induced dose- and time-dependent apoptosis of U937 cells as assessed by flow cytometry analysis, DNA fragmentation, and proteolytic cleavage of poly-(ADP-ribose) polymerase (PARP). Se-MSC increased time- and dose-dependent cytochrome c accumulation in the cytosol, which was greatly inhibited by overexpression of Bcl-2, suggesting that the apoptotic effect by Se-MSC in U937 cells is mitochondrial-dependent. Se-MSC also induced activation of caspases, followed by proteolytic cleavage of PKC-delta. The Se-MSC-induced apoptosis required activities of caspases since pretreatment of a pan-caspase inhibitor z-VAD-fmk greatly suppressed the Se-MSC-induced apoptosis as well as proteolytic cleavage of PKC-delta, suggesting activation of caspases is critical for the Se-MSC-induced apoptosis, and caspases lie upstream of PKC-delta. The Se-MSC-induced apoptosis of U937 cells also required activity of PKC-delta because pretreatment of rottlerin, a specific PKC-delta inhibitor greatly blocked the Se-MSC-induced apoptosis as well as processing and activities of caspases, suggesting activation of PKC-delta is also important for the Se-MSC-induced apoptosis of U937 cells, and PKC-delta lies upstream of caspases. Together, our data suggest the apoptotic mechanism by Se-MSC in U937 cells may be related to cytochrome c release from the mitochondria, and mutual activation between caspases and PKC-delta via a positive feedback mechanism, which may potentiate the apoptotic action by Se-MSC in U937 cells.