We investigated the effects of IL-12 on a murine model of allergic lung inflammation. Administration of IL-12 was timed to interfere with either allergic sensitization (early dosage) or the hypersensitivity inflammatory response in the lung (late dosage), or both (early and late dosages). Comparisons of IL-12- and PBS-treated animals within each treatment group revealed several noticeable effects of IL-12. Early dosage, and the combination of early and late dosages, strikingly decreased ragweed-specific serum IgE, tracheal ring reactivity to acetylcholine, and BAL eosinophilia following allergen challenge. In contrast, late dosage had no effect on IgE levels and only a minimal effect on tracheal ring reactivity, but had a modest effect on recruitment of eosinophils. Early dosage down-regulated IL-5 and IL-10, but did not alter IL-4 or IFN-gamma expression. Late dosage down-regulated IL-5, up-regulated IL-10 and IFN-gamma, but did not change IL-4 expression. The combination of early and late dosage down-regulated IL-4, IL-5, and IL-10 expression, but increased IFN-gamma expression and production in the BAL cells and fluids. Taken together, these results indicate that IL-12 has potent immunomodulatory effects on allergic lung inflammation that depend on the timing of IL-12 administration relative to allergic sensitization and allergen challenge.