Heat-shock proteins (hsps) are induced by many types of stress. In Saccharomyces cerevisiae, a mutation in the HSP104 gene, a member of the highly conserved hsp100 gene family, reduces the ability of log-phase fermenting cells to withstand high temperatures after mild, conditioning pretreatments. Here, we examine the expression of hsp104 and its importance for survival under many different conditions. Hsp104 is expressed at a higher level in respiring cells than in fermenting cells and is required for the unusually high basal thermotolerance of respiring cells. Its expression in stationary phase cells and spores is crucial for the naturally high thermotolerance of these cell types and for their long-term viability at low temperatures. The protein is of critical importance in tolerance to ethanol and of moderate importance in tolerance to sodium arsenite. Thus, the hsp104 mutation establishes the validity of a long-standing hypothesis in the heat-shock field, namely, that hsps have broadly protective functions. Further, that a single protein is responsible for tolerance to heat, ethanol, arsenite and long-term storage in the cold indicates that the underlying causes of lethality are similar in an extraordinary variety of circumstances. Finally, the protein is of little or no importance in tolerance to copper and cadmium, suggesting that the lethal lesions produced by these agents are fundamentally different from those produced by heat.