The cellular tumor antigen p53 is bound to the simian virus 40 (SV40) large T-antigen in SV40-infected and -transformed cells. As a result, p53 can in general be immunoprecipitated by either monoclonal or polyclonal antibodies that react with large T-antigen. Despite extensive immunological characterization of both these antigens, they have not previously been found to share any antigenic determinants. We have isolated several monoclonal antibodies that bind to the human p53 protein (K. Leppard and L. V. Crawford, EMBO J. 2:1457-1464, 1983) and show here that antibody PAb1104 has distinct, intrinsic activities towards both p53 and SV40 large T-antigen. Only a subset of T-antigen is bound by PAb1104. This subset is an oligomeric form of T-antigen, as judged by its sedimentation velocity in sucrose. In contrast, all of the detectable p53 carries the PAb1104-reactive determinant. The detection of a chance cross-reactivity between two antigens that are already well characterized and which associate with one another in vivo is highly unlikely. It is possible therefore that the element of structural similarity between large T and p53 that is implied by our results has some genuine functional significance.