A prototypic “immediate early” gene, c-fos, has been extensively investigated in relation to the differentiation and activation of myelomonocytic cells. The c-fos gene product is associated in transcriptional complexes with the c-jun product. These protooncogenes are part of the regulatory network of gene expression. The present study was designed to investigate expression of the c-jun protooncogene in human circulating myelomonocytic cells. We found that c-jun is constitutively expressed in normal monocytes and granulocytes, whereas low levels of transcripts are found in lymphocytes. Acute myelogenous leukemia (AML) samples of French-American-British Cooperative Group (FAB) subtypes 1 through 4 express appreciable levels of this protooncogene. Normal phytohemagglutinin (PHA)-activated lymphocytes express high levels of c-jun. Expression in normal myelomonocytic cells is detectable even after 18 hours of culture. The c-jun transcripts in myelomonocytic cells have a half-life of approximately 20 minutes and are superinduced by cycloheximide, which affects both the degradation rate of mRNA and the transcriptional activity of the c-jun gene. Functional activation of monocytes and granulocytes with phorbol esters, lipopolysaccharide, and tumor necrosis factor (TNF) increase c- jun expression. This induction is rapid, transient, and does not require intervening protein synthesis. Runoff experiments showed that in freshly isolated untreated monocytes, the c-jun gene is constitutively transcribed, and that induction by lipopolysaccharide is at least in part at the transcriptional level. Moreover, lipopolysaccharide (LPS) treatment reduced the degradation rate of c- jun transcripts, prolonging the half-life to approximately two hours. Expression of c-jun in resting and activated monocytes and granulocytes suggests that this protooncogene may play a role in the differentiation and activation of cells belonging to the myelomonocytic lineage.