Murine polyomavirus (MPyV) has been instrumental in the studies of transformation in vitro as well as for in vivo studies of virus induced tumour development, and for the studies of immune responses against a tumour virus in its natural host. Moreover, it has been extremely useful for unravelling different aspects of DNA replication, transcription, transformation, cell cycle regulation, and assembly of virus-like particles (VLPs). Here, we review how MPyV has also supplied us with information regarding: the molecular nature of a tumour specific transplantation antigen, knowledge on viral persistence and tumour development in its natural host, as well as experience regarding vaccination against infection with a tumour virus.