AbstractThe synergistic cytotoxicity of the combined treatment of hyperthermia and estradiol was demonstrated. This effect was measured for both clonogenic efficiency and membrane function, whose function was expressed as the initial rate of thymidine (dThd) incorporation. V‐79 cells in a suspension culture were simultaneously treated with 10−9‐10−4mg/ml estradiol at temperatures of 41–43°C in a water bath. The survival rate of the V‐79 cells decreased as the temperature increased. However, there was little decrease from treatment by estradiol alone. The survival rate for combined treatment by both heat and estradiol was lower than that of the separate treatment by either hyperthermia or estradiol alone, and the results were statistically significant. The values for Vmax(maximum reaction velocity) of hyperthermia decreased as the temperature increased, although the Vmaxvalue of estradiol treatment alone changed little. The I/Vmaxvalue for the combined treatment of heat and estradiol increased in comparison with that of heat treatment alone. However, the Km(Michaelis constant) value showed little correlation with the survival rate. Based on these results, we propose that estradiol enhances cell membrane damage by