The large T-antigens of SV40 and polyoma virus are nuclear, multifunctional proteins that are essential for replication of the respective viruses. They can also 'transform' cells in culture to varying extents; both can immortalise primary cells, and SV40 large T can additionally induce full transformation. Recently, p105Rb, the protein product of the anti-oncogene RB-1, has been shown to interact with both large T-antigens. SV40 large T-antigen also binds to a p105Rb related protein, p107. SV40 large T differs from that of polyoma virus in its ability to associate with another anti-oncogene product, p53. The significance of these properties to the transforming potential of both viruses is considered.