Two plasmids, one containing the simian virus 40 (SV40) genome and the mouse metallothionein I gene and one containing the v-myc gene of avian myelocytomatosis virus MC29, were coinjected into mouse embryos. Of the 13 surviving mice, one, designated M13, contained both myc and SV40 sequences. This mouse developed a cranial bulge identified as a choroid plexus papilloma at 13 weeks and was subsequently sacrificed; tissue samples were taken for further analysis. Primary cell lines derived from these tissues contained both myc and SV40 DNA. No v-myc mRNA could be detected, although SV40 mRNA was present in all of the cell lines tested. T antigen also was expressed in all of the cell lines analyzed. These data suggest that SV40 expression was involved in the abnormalities of mouse M13 and was responsible for the transformed phenotype of the primary cell lines. Primary cell lines from this mouse were atypical in that the population rapidly became progressively more transformed with time in culture based on the following criteria: morphology, growth rate, and the ability to grow in soft agar and in serum-free medium. The data also suggest that factors present in the mouse regulated the ability of SV40 to oncogenically transform most cells and that in vitro culture of cells allowed them to escape those factors. (5,6,(33)(34)(35)44). Although several viral and cellular genes have been implicated in human tumor formation (15, 39) and many of them transform NIH 3T3 cells in culture (46), it has been difficult to test directly their role in tumor initiation and progression. The introduction of isolated viral and cellular onc genes into intact animals can provide such a test. Increases in tumor incidence in transgenic mice would support a role for the gene in tumor formation, and the spectrum of tissues in which tumors develop would indicate the range of expression of the gene, the action of its product, or both.In this study, two potentially tumor-forming DNA molecules, simian virus 40 (SV40) and the myc gene of avian myelocytomatosis virus MC29, were introduced into mouse embryos. SV40 readily transforms cells in culture (43), although it causes tumors at only a low frequency when either virus or DNA is injected into rodents (1,3,22). Studies with temperature-sensitive and deletion mutants of SV40 showed that the large T antigen is primarily responsible for transformation and that the small t antigen also contributes additional transformation activities (8,38,41).