Apoptosis is a normal physiological process of cell death that can also be experimentally induced by a variety of cytotoxic stimuli. The protein product of the oncogene bcl-2 is an effective inhibitor of apoptosis in mammalian cells, including that caused by exposure to heat. Transient heat resistance can be induced by prior exposure of cells to mild heat treatment. This thermotolerant state is believed to be mediated by an increase in the steady-state concentration of one or several heat shock proteins. Using both morphological and long-term end points, i.e., the appearance of apoptotic cells as assessed by nuclear morphology and clonogenic cell survival, we show that bcl-2 expression confers on thermotolerant cells an even greater resistance to heat-induced cell death. bcl-2 expression does not alter either the steady-state or heat-induced expression of heat shock proteins in cells, nor is bcl-2 itself a stress-inducible protein. The results suggest bcl-2 and the thermotolerant state act via independent mechanisms to inhibit apoptosis and provide evidence that two separate pathways can collaborate to promote cell survival.