Five lines (MKN-28, MKN-45, MKN-74, KATO-III, and HSC-39) of human gastric carcinoma cells were treated with 1 microgram/ml cisplatin for 12, 24, 36, 48, and 72 h. Apoptotic indices (percentages of apoptotic cells) were analyzed at each time point. After incubation with cisplatin, apoptotic cells were detected more frequently among MKN-45 and MKN-74 cells with a wild-type gene for p53 and without expression of the bcl-2 protein than among HSC-39, MKN-28, and KATO-III cells with a mutation or complete deletion of the gene for p53 and with overexpression of the bcl-2 protein. The levels of p53 protein increased after treatment with cisplatin in MKN-74 cells. However, levels of bcl-2 protein were reduced in KATO-III after treatment with cisplatin. Thus, p53 status and the expression of bcl-2 by tumor cells might be good indicators of sensitivity to chemotherapy for patients with gastric cancer.