A heat shock recovery assay on solid medium (Nwaka et al. (1995) J. Biol. Chem. 270, 10193-10198) as well as the classical cell counting method were used to investigate the function of some heat shock proteins in thermotolerance. We show that expression of intact heat shock factor protein (HSF), which regulates the stress induced expression of heat shock proteins (HSPs), is necessary for recovery from heat shock. A HSF1 mutant (hsf1-m3) which does not induce the expression of some heat shock proteins at heat stress (37-40 degrees C) is defective in recovery after heat shock at 50-52 degrees C compared to a corresponding wild-type strain in both stationary and exponentially growing cells. Using two temperature sensitive mutants of the mitochondrial Hsp70 (ssc1-2 and ssc1-3) encoded by the SSC1 gene, we show that the ssc1-3 mutant, which has a mutation in the ATPase domain, is defective in recovery after heat shock in contrast to the ssc1-2 mutant, which has a mutation in the peptide binding domain. Different binding capacities for unfolded proteins are shown to be the molecular reason for the observed phenotypes. The thermotolerance defect of the hsf1-m3 and ssc1-3 mutants is demonstrated for both glucose and glycerol media.