One hundred and forty-eight patients with gastric cancer admitted to Keio University Hospital between July 1988 and October 1992 underwent resection of the primary lesion, as well as single-cell suspension assay of fresh surgical materials with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT assay) for chemosensitivity evaluation. Fifty patients with histologically stage III or IV gastric cancer were enrolled in this study, among whom 10 received no chemotherapy after surgery while 40 received chemotherapy at equivalent dose levels after surgery. The patients given chemotherapy were divided into two groups consisting of an "Adapted" group treated with at least one agent identified as effective by the assay, and a "Non-adapted" group treated with agents to which the cells were not sensitive in the assay, in order to identify the optimal cut-off inhibition rate (IR) in MTT assay for evaluation of the appropriate adjuvant cancer chemotherapy after surgery. A cut-off IR of 30% was optimal for differentiating the survival rates between the "Adapted" and "Non-adapted" groups. Patients treated with drugs which showed more than 30% IR on their surgical specimens showed a better survival rate than patients treated with drugs which were ineffective in the assay.