The objective of this study was to evaluate the negative regulatory role of heat shock protein 70 (HSP70) on endotoxin-induced activation of inflammatory cytokine signaling pathways in a macrophage cell line. Our studies show that elevation of HSP70 either by activation of the heat shock response (HSR) or through forced expression of the hsp70.1 gene downregulates cytokine expression. Our experiments showed that activation of the HSR and HSP70 overexpression could inhibit LPS-mediated expression of the proinflammatory cytokines TNF-α and IL-1 at the mRNA and protein levels. We also investigated the effects of HSP70 elevation on signaling pathways downstream of LPS and its receptors, including the NF-κB and mitogen-activated protein kinase (MAPK) pathways. The effects of HSP70 on cytokine expression were correlated with its effects on activation of NF-κB, a known activator of the tnfα and Il-1 genes. Overexpression of HSP70 inhibited the nuclear translocation of p65, the transcriptionally active component of the NF-κB complex, and prevented the degradation of IκBα, the regulator of NF-κB activity. However, HSP70 elevation did not markedly inhibit signaling through the MAPK arm of the LPS-induced pathway, suggesting that the effects of HSP70 are mediated primarily through the NF-κB cascade. Our experiments therefore suggested that elevated levels of HSP70 inhibit LPS-induced production of inflammatory cytokines by a mechanisms involving inactivation of NF-κB but cast doubt on significant role for the MAPK pathway in these effects.