In Escherichia coli, the efficient export of maltose-binding protein (MBP) is dependent on the chaperone SecB, whereas export of ribose-binding protein (RBP) is SecB independent. To localize the regions of MBP involved in interaction with SecB, hybrids between MBP and RBP in SecB mutant cells were constructed and analyzed. One hybrid consisted of the signal peptide and first third of the mature moiety of MBP, followed by the C-terminal two-thirds of RBP (MBP-RBP112). This hybrid was dependent upon SecB for its efficient export and exhibited a strong export defect in secA mutant cells. A hybrid between RBP and MBP with the same fusion point was also constructed (RBP-MBP116). The RBP-MBP116 hybrid remained SecB independent and only exhibited a partial export defect in secA mutant cells. In addition, MBP species with specific alterations in the early mature region were less dependent on SecB for their efficient export. The export of these altered MBP species was also less affected in secA mutant cells and in cells treated with sodium azide. These results present additional evidence for the targeting role of SecB.