The in vitro succinate dehydrogenase (SD) inhibition (SDI) test was used to determine the heat sensitivity of tumor cell lines and human tumor cells. The tumor cells were exposed to heat in vitro and the decrease in SD activity was assayed using a colorimetric assay, the SDI test. With respect to survival curves of HeLa cells, the SDI test correlated well with the clonogenic assay and the dye exclusion assay. Decrease in the SD activity of HeLa cells after the heat treatment (41-44 degrees C, 10 min to 5 h) depended on both the temperature and the duration of heat exposure. S- and G2/M-phase-rich HeLa cells were more sensitive to heat than were the G1-phase-rich cells. The SDI test exhibited a wide variation in the heat sensitivity among four cell lines (HeLa, B-16, V-79, and a human GT-1 squamous cell carcinoma). Variation in heat sensitivity was also detected among individual tumor tissues obtained from clinical specimens of gastric, esophageal, and colorectal cancers. Gastric cancer tissues were more sensitive to heat than were esophageal and colorectal tissues. We recommended that if the SDI test is used to assess the heat sensitivity of clinical tumor tissues in vitro, appropriate therapy for individual patients can be designed.