Plasmids containing the E1 region of Ad12 DNA can transform certain rodent cells into oncogenic cells. To study the role of the E1a subregion in the process of oncogenic transformation, Ad12 region E1 mutants carrying deletions in the E1a region were constructed. Deletion mutants pR7 and pR8 affect only the 13 S mRNA species encoded by region E1a, whereas deletion mutants pR11 and pR15 damage both the 12 S and 13 S E1a mRNA. All four mutants have lost their capacity to transform primary cultures of baby rat kidney cells, indicating that the E1a gene product encoded by 13 S mRNA is essential for transformation. It was further found that the mutated E1a regions of both pR7 and pR11 can induce expression of region E1b, which implies that the transformation deficiency of these mutants is not due to the inability to activate E1b expression. Surprisingly, the transforming capacity of pR7 and pR11 is restored when these mutant E1 regions are covalently coupled to the SV40 "enhancer" region. Cells transformed by these hybrids plasmids, however, were not tumorigenic in nude mice.