Plasmin, the primary fibrinolytic enzyme, has a broad substrate spectrum and participates in other biological processes dependent upon proteolytic activity. Consequently, plasmin activity is tightly regulated by plasminogen activators and protease inhibitors. In this study, we examined whether regulation of plasminogen gene expression also might provide a new mechanism for controlling this system. We examined the effects of recombinant human interleukin-6 (rhIL-6), a pleiotropic cytokine, on plasminogen mRNA expression in primary murine hepatocytes and Hep3B human hepatoma cells. In primary hepatocytes, rhIL-6 and hydrocortisone separately increased plasminogen mRNA expression, but hydrocortisone did not markedly enhance the response to rhIL-6. Hep3B hepatoma cells exhibited more modest responses to rhIL-6. We used the polymerase chain reaction to amplify a 1,067-bp fragment of the human plasminogen promoter/5' flanking region. This fragment was cloned upstream of a luciferase reporter gene. Hep3B cells transiently transfected with this construct provided approximately 100-fold higher luciferase activity compared to cells transfected with control plasmids, and luciferase activity was increased approximately 4.5-fold when these cells were treated with rhIL-6. Furthermore, mice injected with rhIL-6 exhibited increases in hepatic plasminogen mRNA. Circulating plasminogen levels were significantly higher in the mice injected with rhIL-6 compared to mice injected with saline. Mice injected with lipopolysaccharide (an inducer of IL-6 in vivo) also showed increased hepatic plasminogen mRNA. Thus, plasminogen gene expression can be modulated by rhIL-6, suggesting a new mechanism for regulating biological systems that use plasmin.