Heat-shock protein (Hsp) 70 is an inhibitor of apoptosis and has been shown to protect against nitric oxide-mediated toxicity. To gain mechanistic insights into the actions of Hsp70, we stably transfected RAW 264.7 mouse macrophages with the human Hsp70 gene and investigated critical steps in the progression towards cell demise. Incubation of control and Hsp70-transfected macrophages with S-nitrosoglutathione induced accumulation of the tumour suppressor p53, expression of p21(WAF1/CIP1) (where WAF1 corresponds to wild-type p53-activated fragment 1 and CIP1 corresponds to cyclin-dependent kinase-interacting protein 1) and G(1) cell-cycle arrest. However, cytochrome c translocation to the cytosol and activation of caspase 9 and caspase 3 were markedly reduced in Hsp70-overexpressing cells. In addition, changes in nuclear morphology, as determined by Hoechst staining, and the appearance of cells in the sub-G(1) phase were diminished in Hsp70-overexpressing cells compared with controls. We conclude that, in macrophages, Hsp70 interferes with cytochrome c release from mitochondria and, thereby, prevents nitric oxide-induced apoptosis, but leaves p53 accumulation and interference in the cell cycle intact.