Simian virus 40-transformed cells synthesize high-molecular-weight protein complexes (22 to 30S) that consist of the virus-coded large T antigen (81,500 daltons) and the cellular antigen pp53. These complexes were partially purified from lysates of transformed rat cells by sucrose velocity sedimentation. The stoichiometry of the two proteins in the complex was studied by direct enzyme-linked immunosorbent assays, using alkaline phosphatase-conjugated anti-T and anti-pp53 monoclonal antibodies. The results from these experiments indicate that the T antigen-to-pp53 ratio in the complex is 0.87 +/- 0.27. No statistically significant differences were found in this ratio for faster- and slower-sedimenting complexes. These results from enzyme-linked immunosorbent assays and previous molecular weight estimates of the complex suggest that this complex is composed, on the average, of four molecules of T antigen and four or five molecules of pp53.