Adult rats were exposed to 1 ppm (1.96 mg/cu m) ozone or air for 2 weeks. Animals were sacrificed at 3, 5, 7 or 14 days after the onset of exposure and samples of plasma and lung lavage obtained. Heat inactivated plasma and lavage from animals exposed to ozone, for 5 or 7 days, significantly increased DNA synthesis by lung fibroflasts compared with plasma or lavage from air exposed animals. Fractionation of plasma and lavage samples indicated that the factor responsible had an isoelectric point of 6.45-6.75, and a molecular weight of 38 + or - 3 Kd. This factor has a dose-dependent effect on lung pneumocyte DNA synthesis in culture. It has no effect on cultured fibroblast DNA synthesis, and is distinct from a previously described factor in the plasma of these ozone-exposed animals which enhances fibroblast DNA synthesis. The factor is detectable within 5 days of exposure, and may hold some promise as a marker of early oxidant lung injury. (Copyright (c) 1990 by Hemisphere Publishing Corporation.)