The enterobacterial common antigen (CA) is present in salmonellae of groups B (S. typhimurium) and C(1) (S. montevideo). Mutation at the rfe gene(s), which is required for the biosynthesis of O side chains of the lipopolysaccharide in group C(1) (S-6, 7) but not in group B (S-4, 12), destroys the capacity of the bacteria to synthesize CA. When such mutated group C(1)rfe genes (C-rfe(-)) were introduced into group B strains, the hybrids also became CA(-) and could be restored to CA(+) by introduction of either C-rfe(+) or B-rfe(+) (corresponding genetic region in group B). This indicated the presence of genes for CA synthesis at the rfe site in both groups B and C(1). In rfe(-) mutants of group C(1), which were rough and CA(-), the CA(+) phenotype could be restored by replacing the rfe(-) gene(s) with C-rfe(+). In contrast, B-rfe(+) was able to support the synthesis of trace amounts of CA only, although it was sufficient to restore their ability to synthesize the S-6, 7 side chain of the lipopolysaccharide. Corresponding hybrids (B-rfe(+), C-rfb(+) or C-rfb(-)) were constructed by introducing the C-rfb genes into a group B strain; they also showed only a trace of CA reactivity.