The role of one of the major outer membrane proteins, rOmpB, of spotted fever group rickettsiae was examined. Antibodies generated against native rOmpB inhibited plaque formation by Rickettsia japonica in Vero cells when applied at the time of inoculation of the rickettsiae. However, antibodies to heat-denatured rOmpB did not. Moreover, the soluble recombinant rOmpB also inhibited plaque formation to some extent. Thus it seems that rOmpB functions at least in the adherence of rickettsiae to host cells. To obtain direct evidence of its function in the adherence to and invasion of Vero cells, we generated Escherichia coli transformed by the vector pET-22b(+) inserted with the ompB open reading frame of R. japonica. The recombinant bacteria expressed a 165-kDa protein consistent with the precursor of rOmpB. The protein reacted with monoclonal antibodies to heat-labile epitopes of rOmpB. Immunofluorescence of the recombinant bacteria demonstrated surface expression of the protein. It was shown by light microscopy and transmission and scanning electron microscopy that the bacteria adhered to and invaded Vero cells. Thus, although the recombinant precursor rOmpB was not processed on the outer membrane of E. coli, it functions during these steps. The manner of entry was similar to that of rickettsiae although at a slower rate.