Polyoma virus (PyV) is a murine papovavirus which induces neoplasia via the concerted action of three T- (tumour-) antigens. The lOOkD PyV large T-antigen (LT) is a multifunctional protein involved in several aspects of the lytic or transforming behaviour of the virus. LT is absolutely required for viral DNA replication, and is also a transcriptional regulator, repressing its own transcription, and activating the PyV late transcription unit, as well as several other promoters of viral and cellular origin. LT is able to immortalise primary rodent fibroblasts, co-operates with the PyV middle and small T-antigens to induce full transformation, and reduces cellular growth factor requirements.