A review of published reports showed that there was surprisingly good agreement between different authors on the heat sensitivity of a particular cell line. For several cell lines, there was a marked difference in the heat of inactivation above 43 degrees (deltaH = 148 kcal/mol) and below 43 degrees (deltaH = 365 kcal/mol). This may indicate different mechanisms of cell killing above and below this temperature. With all cell lines tested, M- and S-phase cells were much more heat sensitive than were G1 or G2 cells. The heat sensitivity of S-phase cells is in contrast to the resistance of these cells to X-rays. The effect of hyperthermia on cell progression is discussed. The possibility of greater sensitivity of neoplastic cells to hyperthermia as compared to normal cells seems very promising.