Transforming growth factor β1 (TGF-β1) is a multifunctional, profibrotic cytokine involved in cellular growth and differentiation. We have previously described a cell surface-associated H2O2-generating NADH:flavin:O2 oxidoreductase (referred to as NADH oxidase) activity in human lung fibroblasts induced by TGF-β1 (Thannickal, V. J., and Fanburg, B. L. (1995) J. Biol. Chem. 270, 30334-30338). In this study, the potential for regulation of this novel TGF-β1-activated oxidase in fibroblasts by protein tyrosine phosphorylation was examined. Immunoblots using anti-phosphotyrosine antibody demonstrated a time-dependent but delayed pbosphorylation of two proteins of 115 and 103 kDa in cells stimulated with TGF-β1 (2 ng/ml). Similar to the effect on TGF-β1-induced H2O2 production, phosphorylation of these proteins was blocked by the addition of actinomycin D. The protein- tyrosine kinase inhibitors genistein and herbimycin A inhibited TGF-β1- induced protein tyrosine phosphorylation, NADH oxidase activation, and H2O2 production in a dose-dependent manner. Catalase, diphenyliodonium (an inhibitor of flavoenzymes), and suramin (an inhibitor of receptor activation, added 4 h after TGF-β1) had no effect on the induction of protein tyrosine phosphorylation. Phosphorylation of the 115- and 103-kDa proteins preceded the generation of H2O2 production and returned to control levels when H2O2 was undetectable at 48 h after TGF-β1 exposure. These results suggest that protein tyrosine phosphorylation by a nonreceptor protein-tyrosine kinase(s) regulates the activity of the TGF-β1-responsive H2O2-generating NADH oxidase in human lung fibroblasts. Additionally, this study demonstrates that TGF-β1, which binds to a serine-threonine kinase receptor, is able to induce protein tyrosine phosphorylation in a delayed manner via a signaling pathway that requires transcriptional activation.