The medicinal uses of garlic (Allium sativum) and its constituents have been known for centuries, though its mode of action is still undetermined. Several epidemiological and laboratory studies indicate a potential anti-carcinogenic effect of garlic and some of its constituents. In this study we investigated the effect of aged garlic extract (AGE) and two of its components, S-allylcysteine (SAC), and S-allyl-mercaptocysteine (SAMC) on human breast cancer cells, MCF-7 and MCF-7(ras). Transfection of v-H-ras in MCF-7 cells gives rise to a highly aggressive subset of cells that is estrogen independent and has a five fold greater colony forming efficiency in soft agar than the parent cell line. The modulatory effect of AGE, SAC and SAMC on growth and glutathione cycle was examined in the two cell lines. We noted an anti-proliferative response to SAC and SAMC on both anchorage dependent and independent conditions and an alteration in glutathione level without significant concurrent changes in the glutathione metabolizing enzymes.