In order to determine the most effective anticancer agent for individual human tumor, we have performed several chemosensitivity tests, such as human tumor clonogenic assay (HTCA), succinic dehydrogenase inhibition test (SDI-T), nude mouse isotope assay (NM-IA) and subrenal capsule assay (SRCA). In this study, an novel in vitro chemosensitivity test (ATP-assay) measuring ATP amounts of cancer cells was carried out in 69 fresh gastro-intestinal tumors obtained at surgery. As the results, the evaluable rate of ATP assay was 87.0%. The positive rate of ATP assay against all tumors were 13.3% in mitomycin-C (MMC), 11.7% in adriamycin (ADM), 13.3% in 5-fluorouracil (5-FU) and 18.3% in cis-diamminedichloroplatinum (CDDP), respectively. Overall predictive accuracy rate was 82.8%. The comparative study of the survival rates of the patients with stage IV gastric cancer, receiving sensitive anticancer agents assayed by ATP assay, and those receiving negative anticancer agents revealed that the survival rate of the patients treated with sensitive drugs was longer with Kaplan-Meier analysis. From these results, it seems reasonable to conclude that ATP assay is of value in determining the chemosensitivity of gastrointestinal cancer in each patient.