THE early region of the small DNA tumour virus, simian virus 40 (SV40), is known to code for at least two polypeptides, the t and T antigens (‘small t’ and ‘large T’). Both these polypeptides are expressed in cells transformed by the virus^1–4, and the T antigen has been shown to be essential for both the initiation and maintenance of the transformed state^5–9. We therefore need to know how this T protein interacts with components of the host cell in order to understand the mechanism of SV40-induced transformation. We report here that the T antigen in a line of SV40-transformed mouse cells forms an oligomeric complex with a specific cell coded protein.