Hamster polyomavirus (HaPV) is the causal agent of hair follicle epithelioma in hamsters belonging to a colony bred in Berlin-Buch. These tumors shed virus particles that are assembled in the keratinized layer of the epidermis. By contrast, HaPV induces lymphomas after inoculation into newborn hamsters from a distinct colony bred in Potsdam. These lymphoid tumors accumulate massive amounts of episomal viral genomes characterized by deletions that alter specifically the regulatory and the late coding sequences. Assuming that these alterations of the regulatory region may affect the transcription of the viral oncogenes in the tumor cells, the transcriptional activity of the wild-type and deleted early promoters have been studied in vitro in transient chloramphenicol acetyltransferase (CAT) expression assays. These assays performed in various cell types demonstrate that both versions of the HaPV early promoter carry a weak constitutive activity. Simultaneous expression of the HaPV early gene products leads to a strong stimulation of CAT activity with a concomitant activation of the replication of the plasmid constructs. The results obtained with origin-defective CAT vectors indicate that the replication contributes significantly to the stimulating effect of the early gene products. Indeed, transfection of massive amounts of CAT vectors that are unable to replicate can simulate the dosage effect of replication and also leads to measurable CAT activities. Under these conditions, the wild-type promoter is more active than the deleted version, indicating that sequences within the deletion carry a distinct stimulatory effect on transcription. This conclusion is supported by the observation that the lymphoma cells contain a low level of early transcripts, indicating that the deleted episomal viral templates accumulated in these tumors carry a weak transcriptional activity.