In vitro chemosensitivity was evaluated by bioluminescence ATP assay in 12 human tumors including 7 gastric cancers and 5 colo-rectal cancers. Tumor fragments minced with scissors were exposed to 6 kinds of antitumor drugs: carboquone, adriamycin, aclacinomycin A, mitomycin C, cisplatin and 5-FU at peak plasma concentration or ten times peak plasma concentration. After 3 days at 37 degrees C, each tumor fragment suspension was washed with phosphate-buffered saline, boiled for 3 min and assayed for its intracellular ATP level using the luciferin-luciferase method. The ATP level of the drug-untreated group was 6.01 +/- 4.55 X 10(-10) moles/mg tissue protein in gastric cancers and 9.77 +/- 8.46 X 10(-10) moles/mg tissue protein in colo-rectal cancers. The percentages of cases in which the ATP level was reduced to less than 50% by the antitumor drugs were 70% for carboquone, 11% for adriamycin, 30% for aclacinomycin A, 45% for mitomycin C, 13% for cisplatin and 18% for 5-FU at peak plasma concentration, and 90% for carboquone, 78% for adriamycin, 80% for aclacinomycin A, 82% for mitomycin C, 63% for cisplatin and 82% for 5-FU at ten times peak plasma concentration. The test for chemosensitivity prediction is thus available at peak plasma concentration of antitumor drugs in ATP assay.