The present study was undertaken to elucidate the mechanism for the antifibrotic effect of interferon gamma (IFN-gamma) in the bleomycin (BL)-mouse model of lung fibrosis. The expression of transforming growth factor (TGF-beta) and procollagen I and III and their mRNAs was investigated in the BL-mouse model of lung fibrosis with and without IFN-gamma treatment by Northern and slot blot analyses. Temporal changes in the content of procollagen and TGF-beta mRNAs in the lungs of mice receiving saline or BL by intratracheal route, with and without IFN-gamma treatment by intramuscular route, were quantitated. The level of TGF-beta mRNA increased rapidly and peaked at day 5, whereas the levels of mRNAs for procollagens alpha 1(I) and alpha 1(III) peaked at 10 days after BL instillation. The peak levels of these mRNAs in BL-treated animals were five- to sevenfold higher than those of the control. The increase in TGF-beta mRNA in the lungs of BL-treated mice preceded the increase in the synthesis of type I and type III procollagen mRNAs. BL treatment also increased the hydroxyproline content significantly from 3 to 14 days as compared to the corresponding saline control groups. A maximal increase to 447 micrograms/lung from 223 micrograms/lung in saline control was obtained at 10 days after instillation. Daily treatment with IFN-gamma markedly reduced the BL-induced increases in the mRNA levels of TGF-beta, and procollagen alpha 1(I) and alpha 1(III) without any effect on the lung level of beta-actin mRNA. IFN-gamma treatment also caused significant reduction in the BL-induced increase in the lung hydroxyproline content from 417 to 283 micrograms/lung at 7 days and from 447 to 264 micrograms/lung at 10 days. It may be concluded from the findings of the present study that the cellular mechanisms for the antifibrotic effect of IFN-gamma in the BL-mouse model of lung fibrosis are to initially downregulate the BL-induced overexpression of TGF-beta mRNA, and subsequently procollagen mRNAs, leading to a decreased collagen content.