Heat shock treatment of cells increases their survival and resistance to apoptosis. The kinetics of development of this resistance correlates with the kinetics of synthesis of heat shock proteins (hsps). U937 and Wehi-s cells were cultured for 1 h at 42 degrees C, conditions which induced the synthesis of heat shock proteins 27, 70, and 90. The cells were subsequently permitted to recover for a 2-h period, prior to exposure to the apoptosis inducing agents actinomycin-D (5 micrograms/ml), camptothecin (5 micrograms/ml), and etoposide (25 micrograms/ml). Apoptosis was determined by both DNA fragmentation and flow cytometric analysis. Heat-shocked cultures had a smaller number of apoptotic compared to control cultures when both were exposed to apoptosis inducing stimuli. Transfected Wehi-s cells constitutively overexpressing human hsp 70 or 27 were then examined for their resistance to apoptosis inducing by these drugs. Using the MTT assay, hsp 27 and 70 overexpressing cells exhibited an increased resistance to cell death when compared to the parental line. The parental line demonstrated features of apoptosis, that is, cell shrinkage and single- and double-strand DNA breaks. Taken together these results demonstrate that an increase in cellular levels of hsp 27 or 70, either by a mild heat shock treatment or by stable transfection, increases the resistance of U937 and Wehi-s cells to apoptotic cell death.