The synthesis of prostanoids is regulated by cyclooxygenases (prostaglandin H synthases), which catalyze the conversion of arachidonic acid to PGH 2. Cyclooxygenases are the target of aspirin and other nonsteroidal anti-inflammatory agents. In this study, we found that human polymorphonuclear leukocytes (PMNs) express the inducible isoform of cyclooxygenase, COX-2, when stimulated by LPS whereas the protein was not detectable in freshly isolated human PMNs. We also found by immunohistochemical analysis that COX-2 is expressed in PMNs in inflamed human tissues. COX-2 was induced in a time-and concentration-dependent fashion when isolated human PMNs were exposed to LPS; COX-2 was also induced, or its expression was increased, by TNF-α, IL-1, and IL-8. Expression of COX-2 in stimulated PMNs was paralleled by secretion of PGE 2. The release of PGE 2 was blocked by a selective nonsteroidal inhibitor of COX-2, indicating that the enzyme is responsible for the prostanoids produced, and was inhibited by dexamethasone. The time course of LPS-induced COX-2 expression and other features were different in freshly isolated PMNs, monocytes, and macrophages, indicating that COX-2 expression is differentially regulated in myeloid cells of different lineages and degrees of maturation. Consistent with this, IL-4 and IL-10, which suppressed LPS-induced COX-2 expression in monocytes, had little effect on this response by PMNs. These experiments demonstrate that PMNs express COX-2 when appropriately stimulated. Thus, they may actively influence the eicosanoid composition of the acute inflammatory milieu.