We characterized thermal behaviours of cellular components by differential scanning calorimetry (DSC) in order to investigate how Saccharomyces cerevisiae cells acquire thermotolerance after heat shock or in stationary phase. Whole-cell DSC pro®les consisted of at least ®ve endothermic components over the range 45±75uC for exponentially growing, heat-shocked and stationary-phase cells. In these pro®les, we attempted to localize the endothermic pro®les due to denaturation of the two critical targets which were predicted by using the Arrhenius parameters of hyperthermic killing of the cells (Obuchi et al., 1998). This prediction indicated that (a) the heat shock stabilized one family of targets and destabilized the other, while (b) arrest in stationary phase stabilized both targets. Therefore, the heat-shock response does not stabilize all cellular components, and arrest in stationary phase appears to stabilize cellular components in a different manner from the heat-shock response. It was not possible unambiguously to resolve the pro®les of the critical targets in the DSC scans of whole cells. Components I (T m =49.7uC) and II (T m =56.1uC) may both include denaturations of critical targets 1 (T m =55.4uC) and 2 (T m =53.0uC) in exponential cells. Components I and II were both stabilized (T m =53.5 and 57.2uC, respectively) in heat-shocked cells. Subcellular fractions suspended in 1.2 M trehalose solution, which mimics the cytosol in tolerant cells, were more stable than those in 0.6 M KCl, which mimics the cytosol in sensitive cells. The microsomal fractions in KCl and trehalose had endothermic pro®les in similar temperature ranges to those predicted for sensitive and tolerant cells, respectively. This agreement suggests that the microsomal fraction may contain critical targets, and that trehalose accumulation in the heat-shocked and in the stationary phase yeast cells is a stabilizer of cellular components.