We isolated 23 marine actinomycetes from seawater samples. Of these, strain SY-103 exhibited the strongest cytotoxic activity on human leukemic cell lines. Biochemical tests and 16S rDNA sequencing of this strain allowed us to identify SY-103 as a strain of the genus Streptomyces. In the present study, the pure cytotoxic compound (PCC) from Streptomyces sp. SY-103 metabolites was purified by reverse-phase HPLC and the biochemical mechanisms of PCC-induced apoptosis in cultured human leukemic cell lines were investigated. The exposure of cells to PCC resulted in growth inhibition and induction of apoptosis, which was associated with the proteolytic activation of caspase-3 and down-regulation of anti-apoptotic Bcl-2 protein. However, PCC-induced caspase-3 activation and apoptosis were significantly attenuated in Bcl-2 overexpressing U937 cells. z-DEVD-fmk, a caspase-3 specific inhibitor, blocked caspase-3 activation and increased the survival rate of PCC-treated U937 cells. The activity of Akt was also inhibited in PCC-treated cells, and phosphatidylinositol 3-kinase (PI3K)/Akt inhibitor, LY294002, sensitized the cells to PCC-induced apoptosis indicating that the down-regulation of the Akt signaling pathway plays a key role in PCC-induced apoptosis. Our findings imply that some of the biological functions of Bcl-2 and PI3K/Akt are attributed to their ability to inhibit PCC-induced apoptosis; therefore, it is suggested that this compound is a promising anti-cancer agent for leukemia cells.