The TolA, TolB, TolQ, and TolR proteins are involved in maintaining the integrity of the Escherichia coli outer membrane and in the import of group A colicins and filamentous phage DNA. TolA, TolQ, and TolR are localized in the inner membrane while TolB is periplasmic, although a small amount of membrane-associated TolB is always found. In vivo cross-linking experiments with formaldehyde were performed in order to determine the proteins interacting with TolA. In wild-type strains, two specific complexes of 65 and 71 kDa, comprising TolA, were identified. These complexes were absent in a tolQ strain, while only the 65-kDa complex was absent in a tolR strain. When the tol strains were transformed with plasmids encoding TolR or TolQ, the specific complexes were restored. Moreover, immunoprecipitation experiments with the antiserum directed against TolA indicated that TolQ and TolR were co-immunoprecipitated with TolA after cross-linking. These data demonstrate that TolA interacts directly with TolR and TolQ. Two truncated TolA proteins devoid of either the C-terminal or the central domains of the protein were subjected to in vivo cross-linking. Since these two TolA derivatives still formed specific complexes with TolR derivatives still formed specific complexes with TolR and TolQ, we concluded that the TolA N-terminal domain interacted with these proteins.