The papovavirus SV40 is able to induce tumours in susceptible hosts and will transform cells in vitro. Its major early protein, large T antigen, is required for viral DNA synthesis, both in vivo and in vitro, and is also responsible for the oncogenic action of the virus. We have made use of an extensive library of anti-T monoclonal antibodies to investigate the cellular effects of T. Large T shares an antigenic determinant with a growth-regulated host protein, p68, which is a member of an expanding super-family of helicases with particular homology to the translation initiation factor elF-4A. We have also studied the binding and interaction of large T with two particular host components: the replicative enzyme DNA polymerase alpha and the proto-oncogene p53. These two proteins bind to similar regions of T and exert similar effects on its antigenic structure. We found that p53 can block the binding of DNA polymerase alpha to T as well as co-existing with DNA polymerase alpha in a trimeric complex with T. This suggests that these interactions may be important in the oncogenic and replicative action of large T.