Hyperoxia has deleterious effects on lung form and function, however, the chain of molecular events that occur as a result of oxygen exposure remain unclear. The influx of macrophages to the lung is an important early component to the lung's response to excess oxygen, and we have previously found a hyperoxia-induced factor secreted by cultured macrophages which, when applied to cultured pulmonary epithelial cells, stimulates the expression of manganese superoxide dismutase. We therefore hypothesized that exposure of macrophages to hyperoxia would result in a change in gene expression which could be detected by differential display PCR. We exposed RAW 264 cells to ≥ 95% oxygen for 24 hours, harvested RNA from these cells and from control cells maintained in ambient oxygen, performed RT-PCR using pairs of random primers, and displayed the PCR products on a 6% sequencing gel. A band upregulated by hyperoxia was identified, cut out and reamplified, resulting in a single band of 180 bp, visualized by gel electrophoresis. Verification of differential expression of mRNA was done by Northern Analysis, using the reamplified, labeled, differentially expressed band as a probe. A mRNA of approximately 2200 bases was identified which was reproducibly upregulated by hyperoxia, as well as by LPS and interferon gamma. The differentially expressed PCR product was cloned and sequenced, revealing a product with 99% identity to mouse urokinase mRNA (2299bp). Urokinase activates the transformation of plasminogen to plasmin, which then promotes the transformation of fibrin to fibrin split products. Urokinase can also be instrumental in restructuring other extracellular matrix molecules such as collagen IV, laminin, fibronectin and chondroitin sulfate, and urokinase activity levels have been inversely correlated with severity of disease in premature infants (Am Rev Respir Dis 1992, 146:492). We speculate that one important function of the macrophages recruited to the lung following a hyperoxic injury is to secrete urokinase.