Transforming growth factor alpha (TGF-alpha) has been shown to induce liver tumors within 1 year in transgenic male mice in which this potent mitogen is overexpressed. To determine more precisely how TGF-alpha participates in multistep tumorigenesis of the liver, genotoxic (diethylnitrosamine or dimethylnitrosamine) and nongenotoxic (phenobarbital) chemical carcinogens were administered independently to TGF-alpha transgenic mice [line MT42 on a Crl:CD-1(ICR)BR background]. TGF-alpha overexpression dramatically accelerated carcinogen-induced hepatocarcinogenesis in MT42 males but not females. Interestingly, all three chemical agents were found to enhance strongly both hepatic tumor formation and progression in TGF-alpha transgenic male mice. In this study 100%, 90%, and 78% of transgenic males exposed to diethylnitrosamine, dimethylnitrosamine or phenobarbital, respectively, developed tumors between 24 and 32 weeks of age. Moreover, approximately 70% of tumor-bearing transgenic mice from each treatment group had hepatocellular carcinomas; no malignant lesions were found in any carcinogen-treated or untreated nontransgenic mice or in untreated MT42 mice at this age. These results demonstrate that chemical agents as diverse as nitrosamines and phenobarbital act as cocarcinogens with TGF-alpha in the livers of these transgenic mice, indicating that TGF-alpha possesses the unique ability to complement both initiation and promotion in hepatocarcinogenesis. Furthermore, because carcinogen-induced malignant conversion was restricted to transgenic mice, constitutive TGF-alpha overexpression may promote liver tumor progression as well.