MK615 is an extract mixture containing hydrophobic substances from Japanese apricot. In this study, the antineoplastic effects of MK615 against breast cancer cells were investigated. Two breast cancer cell lines, MDA-MB-468 (MDA) and MCF7, were cultured with (600, 300, and 150 mug/mL) or without MK615. After 72 hours of incubation, growth inhibition was evaluated by MTT assay. The cells were then cultured with MK615 (300 mug/mL) and morphological changes were studied by light and electron microscopy. Finally, the mechanism of the antineoplastic effect of MK615 was evaluated by cell cycle and apoptosis assay. MK615 inhibited the growth of MDA and MCF7 in a dose-dependent manner. The percentage growth inhibition of MDA at dosages of 600, 300, and 150 mug/mL was 59.2%, 52.4%, and 23.3%, respectively, and that for MCF7 was 83.5%, 52.7%, and 16.6%, respectively. Morphological changes after MK615 treatment included massive vacuolization in the cytoplasm and apoptotic changes in the nucleus. These changes began to be apparent after at least 6 hours of incubation. Cell cycle analysis showed that MK615 increased the proportion of cells in the G2-M phase in both MDA (7.8-11.7%) and MCF7 (8.1-18.7%), and finally both cell lines became apoptotic. The proportion of apoptotic cells increased with incubation time. MK615 effectively inhibits the growth of breast cancer cells in vitro, possibly by cell cycle modification and apotosis induction. MK615 should be further investigated as a promising anti-breast cancer agent.