Use of genetically engineered mice offers a unique approach to identifying and investigating factors that may influence tumor development. We have used conventional histopathologic and ultrastructural techniques to characterize lung tumors in three lines of transgenic mice bearing an albumin enhancer/promoter linked to a mutated human H-ras gene. Mice in all three lines developed multiple alveolar-bronchiolar (A/B) adenocarcinomas that are eventually lethal. The large diversity in tumor morphological features and differential tumor growth rates suggests that secondary events contribute to tumor phenotype and biological behavior. Two of the transgenic lines developed numerous A/B neoplasms within 6 to 8 weeks and thus may be useful animal models for testing potential anticancer chemotherapeutic agents. The other line lived for approximately 10 months, had fewer A/B tumors, but also developed bronchiogenic tumors. All three transgenic lines may be useful models for studying factors that affect lung tumor development.