The structure of the D2 protein, a polypeptide structurally and functionally related to the simian virus 40 large T antigen, was examined by electron microscopy. The observed size of the protein indicates that it most commonly exists as a tetrameric assembly of its 107,000 M r monomer. In addition to the tetramers, both monomers and dodecamers are seen. Binding of D2 protein to a fragment of simian virus 40 DNA was found to occur preferentially at the origin of replication. The major binding species of the protein is the tetramer. © 1981.