A subset of Escherichia coli proteins, in particular cofactor-binding proteins with so-called twin-arginine signal peptides, is transported to the periplasm via the twin-arginine translocation (Tat) pathway. The tatA and tatB genes encode important components of the export system and we have analysed whether the proteins encoded by these genes physically interact. Using co-immunoprecipitation experiments, we show that TatA and TatB do indeed associate with each other. Gel filtration chromatography demonstrates that both proteins are present in a large complex with an apparent molecular mass of approximately 600 kDa, indicating the presence of other components and/or several TatA and TatB subunits. Finally, we show that TatA is stable in the absence of TatB and may participate in a separate complex lacking TatB in wild-type cells.