The effects of supranormal temperatures on normal human cells (derived from normal adult and embryonal tissues) and neoplastic human cells (derived from biopsies of malignant tumors) were quantitatively studied in tissue culture. Melanoma cells were compared with melanocytes derived from fetal uveas. Colon carcinoma cells were compared with embryonal intestinal epithelial cells. Malignant neuroepithelial cells derived from a teratocarcinoma of the ovary were compared with neuroepithelial cells derived from fetal brain. Fibrosarcoma cells were compared with normal adult fibroblasts. All cells defined as neoplastic produced malignant tumors when injected into nude thymus-deficient mice at doses of 1 .times. 107 cells or less. Exposure to temperatures of 42.5-43.0.degree. C for 4-8 h had a significantly greater lethal effect on tumor cells than nonneoplastic cells.