Sch 40120 (10-(3-chlorophenyl)-6,8,9,10-tetrahydrobenzo[b] [1,8]naphthyridin-5(7H)-one) is an inhibitor of the 5-lipoxygenase enzyme in rat neutrophils, human neutrophils and the the MC9 murine mast cell clone with IC50 values of 8, 4 and 7 microM, respectively. The drug was examined for its effects on acute inflammatory responses in the paw and pleural cavity of rats. The drug suppressed paw inflammation triggered by a reverse passive Arthus reaction or a subplantar injection of the polysaccharide carrageenan with p.o. ED50 values of 0.2 and 1.5 mg/kg, respectively. In reverse passive Arthus reaction and carrageenan pleurisy models, Sch 40120 was found to suppress both the cellular and fluid components of the acute inflammation. The p.o. ED50 values for inhibition of cells and fluid in pleurisy models were in the range of 0.1 to 0.7 mg/kg. When applied locally to the ears of mice, the drug blocked an arachidonic acid-induced and leukotriene-mediated ear inflammation with an ED50 of 0.072 mg/ear. These findings suggest that Sch 40120 is a potent anti-inflammatory agent that may be particularly useful in the treatment of inflammatory diseases such as psoriasis in which leukotrienes appear to be major mediators of the pathological symptoms that characterize the disease state.