Cell transformation leads to alterations in both transcription and DNA replication. Activation of transcription by the expression of a number of transforming oncogenes is mediated by the transcription factor AP1 (Herrlich & Ponta, 1989; Imler & Wasylyk, 1989). AP1 is a composite transcription factor, consisting of members of the jun and fos gene-families. c-jun and c-fos are progenitors of oncogenes, suggestion that an important transcriptional event in cell transformation is altered activity of AP1, which may arise either indirectly by oncogene expression or directly by structural modification of AP1. We report here that the v-jun oncogene and its progenitor c-jun, as fusion proteins with the lex-A-repressor DNA binding domain, can activate DNA replication from the Polyoma virus (Py) origin of replication, linked to the lex-A operator. The transcription-activation region of v-jun is required for activation of replication. When excess v-jun is expressed in the cell, replication is inhibited or 'squelched'. These results suggest that one consequence of deregulated jun activity could be altered DNA replication and that there are similarities in the way v-jun activates replication and transcription.