TolA, -B, -Q, and -R proteins are involved in maintaining the cell envelope integrity of Escherichia coli; they have been parasitized by the group A colicins and the single strand DNA of some filamentous bacteriophages to permit them to enter the cells. TolA and TolR are anchored to the inner membrane by a single transmembrane domain, TolQ is an integral membrane protein with three transmembrane segments, and TolB has recently been found to be periplasmic although it is partially membrane-associated. The latter result suggests that TolB might interact with membrane proteins. Other lines of evidence favor the existence of a Tol complex. To further characterize this complex, we investigated which proteins interact with TolB. For this purpose, two different methods were used. First, we took advantage of the existence of a tagged TolB (TolBep) to perform immunoprecipitation under native conditions in order to preserve the putative associations of TolBep with other proteins. Secondly, in vivo cross-linking experiments with formaldehyde were performed. These two approaches led to the same result and demonstrated for the first time that a component of the Tol system, TolB, interacts with a protein located in the outer membrane, the peptidoglycan-associated lipoprotein.