His+ hybrids from a cross between a Salmonella typhimurium donor and an Escherichia coli O8 recipient expressed E. coli O8 specificity and in addition Salmonella O4,12-specificity. This indicated that the recipients had received the his-linked donor rfb cluster determining the synthesis of S. typhimurium O-specific repeat units and that the rfb genes of both mating partners are functional in these hybrids. Chemical analyses showed that the hybrids contained an E. coli O8 lipopolysaccharide (O antigen) and a S. typhimurium specific lipopolysaccharide with only one O-specific repeat unit (SR antigen). O8-negative mutants selected from the O8-positive hybrids retained the Salmonella O-specificity and represent semi-rough (SR) forms, because the rfc gene(s) determining the polymerization of repeat units has not been transferred. Attempts to introduce the S. typhimurium rfc locus into E. coli O8 remained unsuccessful. Crosses between a S. typhi donor and E. coli O8 gave rise to smooth (S) and SR His+ recombinants exhibiting only S. typhi O-specificity. The smooth recombinants are assumed to have obtained the his-linked rfb cluster and in addition the rfc gene(s) of the donor. The exchange of the rfb region of such smooth recombinants by that of a S. typhimurium donor led to smooth hybrids with O4,(5), 12-specificity. The phenotypically smooth recombinants exhibited concomitantly S- and SR-lipopolysaccharides of S. typhi and S. typhimurium O-specificity, respectively.