Epidemiological and experimental carcinogenesis studies provide evidence that certain components of garlic have anti-cancer activity. Although the biotransformed garlic derivative S-allylmercapto-L-cysteine (SAMC) has been reported to show an inhibitory effect on tumorigenesis, the mechanisms are poorly understood. The present study investigated the effect of SAMC on the growth of human gastric cancer SNU-1 cells. Upon treatment with SAMC, a concentration-dependent inhibition of cell proliferation was observed and cells developed many of the hallmark features of apoptosis, including DNA fragmentation and an increase in the sub-diploid population. The anti-proliferative and apoptotic effect of SAMC was associated with the induction of Bax, p53, and caspase-9, rather than the induction of Bcl-2 and p21. Mitochondrial cytochrome c activation and an in vitro caspase-3 activity assay demonstrated that the activation of caspases accompanies the apoptotic effect of SAMC, which mediates cell death. These results suggest that the apoptotic effect of SAMC on gastric cancer SNU-1 cells may be connected with caspase-3 activation through the induction of Bax and p53, rather then Bcl-2 and p21.