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, from animals exposed to ozone for 7 or 14 days, significantly increased DNA synthesis by lung fibroblasts compared with plasma 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 32 +/- 2 Kd. This factor has a dose-dependent effect on lung fibroblasts in culture, but no significant effect on cultured pneumocyte DNA synthesis. The factor is detectable within 72 hours of exposure, and may hold some promise as a marker of early oxidant lung injury. (Copyright (c) 1989, Hemisphere Publishing Corporation.)