In this study we have examined the expression of a heat-shock protein (HSP) 70 gene in normal human peripheral blood leukocytes. Northern blot analysis showed that appreciable levels of hsp70 mRNA are present in monocytes and granulocytes, whereas transcript levels were barely detectable or absent in lymphocytes. Monocytes functionally activated by bacterial lipopolysaccharide (LPS) showed an early (15 minutes) increase of hsp70 transcripts that was shown, by actinomycin D blocking and nuclear run-off experiments, to be dependent on transcriptional activation of the gene. LPS did not appreciably affect the hsp70 mRNA half-life. Monocytes exposed to inactivated streptococci, phorbol-12- myristate-13-acetate, and tumor necrosis factor showed augmented levels of hsp70 transcripts, whereas interferon-gamma and monocyte, granulocyte, and granulocyte-monocyte colony-stimulating factors had no effect. Adherence to plastic augmented hsp70 expression in monocytes. S1 protection analysis indicated that the gene expressed in monocytes is indeed a heat-inducible member of the hsp70 gene family rather than a constitutively expressed heat-shock cognate gene. Western blot analysis showed that a heat-inducible HSP72 was present in monocytes and, at augmented levels, in LPS-treated monocytes. LPS-activated monocytes were more resistant to heat shock than unstimulated cells. These data indicate that a heat-inducible hsp70 gene can be efficiently expressed in myelomonocytic cells at physiologic temperatures. Expression of hsp70 genes in monocytes suggests a possible role of heat- inducible genes in the differentiation and/or functional activation of terminally differentiated nonproliferating elements of the myelomonocytic lineage.