Cytokines released from monocytes and macrophages are major mediators of inflammation. Heat shock significantly inhibits cytokine production from these cells. To investigate whether this inhibitory effect was mediated by heat-shock proteins (HSP), we transfected human peripheral blood monocyte-derived macrophages (MDM) with HSP-70 cDNA and examined Brucella melitensis lipopolysaccharide (LPS)-induced cytokine production in transfected cells. Over-expression of HSP-70 protein in the gene-transfected MDM had no effect on cytokine synthesis unless LPS was added. LPS-induced increases in production of tumour necrosis factor alpha (TNF-alpha), interleukin 1beta (IL-1beta), IL-10 and IL-12 were significantly inhibited by the over-expression of HSP-70. However, over-expression of HSP-70 did not block LPS-induced increase in IL-6 synthesis. To further confirm these results, an antisense HSP-70 DNA oligomer was used to block HSP-70 synthesis. The inhibitory effect of HSP-70 on LPS-induced cytokine production in gene- transfected cells was completely reversed after treatment of cells with 5 microM antisense HSP-70. The same concentration of antisense HSP-70 also partially reversed heat-shock-induced inhibition of LPS-stimulated cytokine production. These results suggest that HSP-70 is involved in the regulation of LPS-induced cytokine production and that this family of proteins plays a role in mitigating adverse effects of endotoxin during infection or other pathological stresses.