Effects of antifebrile (sulpyrine) and anticancer (5-fluorouracil) drugs on the growth of several cultured human cells at high temperature were investigated. Cultured normal human vascular endothelial (HVE) and normal human fetal lung (HAIN-55) cells at 37.2℃ were sensitive to sulpyrine, and their sensitivities to the drug were markedly enhanced when they were incubated at 41.5℃. In contrast, sensitivity of malignant human cells (HeLa cells) to sulpyrine was not found at 37.2℃, however sensitivity of the cells to the drug was manifested at 41.5℃ of incubation. There was no effect of 5-fluorouracil (5-FU) on the growth of HVE and HAIN-55 cells at 41.5℃, while HeLa cells showed high susceptibility to this drug at the same temperature. The results suggest the possibility that normal human cells may be sensitive to antifebrile drugs but not to anticancer drugs at high temperature, whereas malignant human cells may be susceptible to both antifebrile and anticancer drugs at high temperature.