In the adenovirus type 12 (Ad12) hamster cell system, abortive virus infection is one of the factors associated with the highly efficient oncogenesis in newborn Syrian hamsters. We have shown earlier that the replication and efficient late transcription of the Ad12 genome are blocked in Syrian hamster cells. Some of the early Ad12 functions are transcribed in these cells, although at a minimal rate. In the present study, we demonstrate that low expression levels of the E1A and precursor to terminal protein (pTP) genes of Ad12 seem to be responsible for the lack of Ad12 DNA replication in hamster cells. The Ad12 genes for the E1A functions or for pTP were tethered to the strong early promoter of the human cytomegalovirus and transfected into BHK21 cells. Subsequently, these cells were infected with Ad12 virions. In Ad12-infected BHK21 cells, which overexpressed pTP or E1A, full-length Ad12 DNA was de novo synthesized, as documented by metabolic labeling with [³H]thymidine and by zone velocity sedimentation in alkaline sucrose gradients followed by gel electrophoresis of the ³H-labeled DNA and Southern blot hybridization to ³²P-labeled Ad12 DNA. Transfection of the cloned E1A region of Ad2 yielded similar results. The newly synthesized Ad12 DNA was covalently linked to pTP. The Ad12 DNA binding protein (DBP) and DNA polymerase (pol) genes were transcribed at levels similar to those in merely Ad12-infected cells. In pTP or E1A gene-transfected and Ad12-infected BHK21 cells, marginal levels of late Ad12 mRNA were detectable. Late Ad12 proteins were, however, not synthesized. Apparently, Ad12 DNA replication in hamster cells is rendered impossible due to insufficient threshold levels of the viral E1A and/or pTP.