Several genes for the export of protein in E.coli, including secA, secB, secE, secY, have been identified by genetic approach. Most genes except secB are essential for cell viability. SecB, howeve, is nonessential and plays a role as a chaperone in protein export. It involves in the export of only a subset of proteins. Previous studies reported that secB::Tn5 strain did not show any export defect of ribose-binding protein(RBP), thus RBP is regarded secB-independent. Here it is shown that the export of RBP requires intact sec genes, and even secB in abnormal physiological conditions. At nonpermissive temperature, both secA 51(Ts) and secY24(Ts) strains showed the accumulation of precursor RBP suggesting that the SecA and SecY involved in the export of ribose-binding protein. Moreover, prlA(secY) and prlD(sedA) strains were able to suppress a signal sequence mutation, rbsB103. SecB, however is not absolutedly required for the export of wild type, but, is necessary for the slowly exporting mutants. All RBPs with defective signals tested were weakly secB-dependent. In a situation where cells were treated with uncoupler, CCCP, the export of wild-type RBP was also secB-dependent. When SecB was present, nonfunctional GroEL did not affect the export of RBP. These results suggest that at least in the posttranslational translocation, SecB participates in the export of RBP.