To examine the involvement of heat shock proteins in the induction of thermotolerance in Chinese hamster V79 cells, thermotolerance was induced by heating of the cells at 42 degrees C for 4 h or at 44 degrees C for 20 min, or by treatment of the cells with 50 microM sodium arsenite for 3 h or 20 micrograms/ml puromycin for 4 h. Under unstressed conditions V79 cells synthesized constitutively three major heat-shock proteins, hsp70, hsp85 and hsp105. On exposure to conditions under which thermotolerance was induced, the synthesis of constitutive hsp70, hsp85 and hsp105 increased, but the inducible form of hsp70 was not synthesized, indicating that this inducible form was not necessary for the induction of thermotolerance. Although the amounts of heat-shock proteins synthesized in the cells that acquired thermotolerance were not always more than those synthesized constitutively in unstressed cells, the stressed cells synthesized heat-shock proteins (especially hsp70) preferentially over other proteins. As the level of hsp70 in the thermotolerant cells was almost the same as that in unstressed cells, the specific accumulation of hsp70 seemed not to be required for the acquisition of thermotolerance. From these findings it seemed likely that, for the induction of thermotolerance in V79 cells, hsp70 preferentially synthesized during or after the stress has an important function. Or the synthesis of heat shock proteins may not be important, and constitutively synthesized heat-shock proteins acquire a specific function during or after the stress.