Allylmercapto glutathione S-conjugate, S-allylmercapto-L-cysteine (SAMC), which is biotransformed from allyl sulfides and from naturally occurring water-soluble garlic derivatives, has been known to inhibit tumorigenesis. We found that SAMC was able to induce apoptosis in gastric cancer cells in vitro. We report that SAMC inhibited tumor growth rate by 31.36% and 37.78% at doses of 100 and 300 mg/kg, respectively. Apoptosis in the implanted tumor cells was manifested by apoptotic characteristics, including morphological changes of chromatin crescent, cell shrinkage and membrane blebbing. The apoptosis index of 100 mg/kg and 300 mg/kg of SAMC was 20.74±2.50% and 30.61±2.42%, respectively, by terminal deoxy-nucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) nick-end labeling (TUNEL) staining. The positive rate of B-cell lymphoma 2 (bcl-2) protein expression of control, 100 mg/kg SAMC and 300 mg/kg SAMC was 15.20±1.67%, 10.94±1.57%, and 8.24±1.07%, respectively, by immunohistochemical staining. The positive rate of bax protein expression of control, 100 mg/kg SAMC and 300 mg/kg SAMC was 15.30±1.90%, 23.18±1.81%, and 25.26±3.03%, respectively. We also observed decreases in bcl-2 mRNA and increases in bax mRNA by SAMC in a dose-dependent manner by reverse transcription-polymerase chain reaction (RT-PCR). These results suggest that SAMC may regulate bcl-2 and bax to induce apoptosis in transplanted tumor cells.