To study the mechanism by which SV40 large T antigen transforms cells under physiological conditions, we analysed several mutant forms of T antigen for their ability to induce cell proliferation and tumorigenesis in transgenic mice. These mutant proteins, which differ in their ability to form complexes with the tumor suppressors pRB and p53, were analysed under conditions in which wild-type T antigen induces choroid plexus papillomas as a result of uniform proliferation of the entire choroid plexus epithelium. The results presented here show that binding of T antigen to p53 is not required for induction of choroid plexus tumors. However, tumorigenesis does appear to require the binding of T antigen to pRB/p107. An additional activity, resident in the amino-terminal one-fifth of the protein, may also play a role. These experiments indicate the importance of whole-animal assays in determining the molecular basis of transformation, since each of these mutants possessed similar transformation phenotypes in culture but showed distinct phenotypes in the choroid plexus of the animal.