v-Ha-ras transgene abrogates the initiation step in mouse skin tumorigenesis: effects of phorbol esters and retinoic acid. Experimental carcinogenesis has led to a concept that defines two discrete stages in the development of skin tumors: (i) initiation, which is accomplished by using a mutagen that presumably activates a protooncogene, and (ii) promotion, which is a reversible process brought about most commonly by repeated application of phorbol esters. We have created a transgenic mouse strain that carries the activated v-Ha-ras oncogene fused to the promoter of the mouse embryonic alpha-like, zeta-globin gene. Unexpectedly, these animals developed papillomas at areas of epidermal abrasion and, because abrasion can also serve as a tumor-promoting event in mutagen-treated mouse skin, we tested these mice for their ability to respond to phorbol ester application. Within 6 weeks virtually all treated carrier mice had developed multiple papillomas, some of which went on to develop squamous cell carcinomas and, more frequently, underlying sarcomas. We conclude that the oncogene "preinitiates" carrier mice, replacing the initiation/mutagenesis step and immediately sensitizing them to the action of tumor promoters. In addition, treatment of the mice with retinoic acid dramatically delays, reduces, and often completely inhibits the appearance of promoter-induced papillomas. This strain has use in screening tumor promoters and for assessing antitumor and antiproliferative agents. 