When nuclei were isolated from exponentially growing HeLa S3 cells immediately after a treatment with hyperthermia and/or procaine-HCl, an increase in nuclear protein binding was observed. The extent of this increase, however, did not correlate with cell survival under all conditions of the various treatments. For example, an increase up to 40 per cent in nuclear protein binding as a result of procaine treatment did not lead to a decrease of survival, while a 40 percent increase of nuclear protein binding as a result of hyperthermia corresponded with over 90 per cent cell killing. In addition the extent of heat-induced enhancement of nuclear protein content was approximately equal for thermotolerant and heated control cells, or for cells heated in the presence of procaine. The rate of decay in nuclear protein binding upon post-heat incubations at 37 degrees C of the cells, however, was enhanced in tolerant cells and retarded in cells heated in the presence of procaine as compared to heated control cells. These results show that, in spite of suggestions in other reports, neither the initial rate of enhanced protein binding nor the extent of the protein bound to the nucleus seems a reliable measure for heat toxicity. The capacity of the cell to reverse this heat-induced protein binding must be considered.