The TraT protein is a surface-exposed lipoprotein, specified by plasmids of the IncF group, that mediates serum resistance and surface exclusion. The structure and functions of the TraT protein determined by plasmid R6-5 were probed by genetic insertion of a foreign antigenic determinant, the C3 epitope of poliovirus, at residues 61, 125, 180, 200 or 216 of the protein. The chimeric proteins were transported to the outer membrane and, in three cases, immunoassays with an anti-C3 monoclonal antibody indicated that the C3 epitope was exposed on the cell surface. Three of the hybrids, with insertions at residues 125, 180 and 200, assembled into the trypsin-resistant oligomeric form characteristic of the wild-type protein suggesting that these regions are not involved in TraT subunit: subunit interactions. The hybrid protein carrying the C3 epitope at position 180 retained surface exclusion activity and functioned in a genetic suppression assay indicating that its localization, folding and organization was not grossly altered from that of the wild-type protein. However, none of the proteins mediated serum resistance. These results suggest that the surface exclusion and serum resistance activities of the TraT protein may be functionally separable. Applications of the protein for the transport of foreign antigenic determinants to the cell surface are discussed. Additionally, from cloning and sequencing of traT from the IncFII plasmid Co1B-K98, the region of TraT protein responsible for specificity of surface exclusion was located and confirmed experimentally.