A set of R (rough) mutants, in which the synthesis of the cell wall lipopolysaccharide is defective, were analyzed genetically. These same Salmonella minnesota mutants have been used extensively in biochemical studies. By combining the information from all these studies, we obtained a complete genetic description of many of these mutants. Most of them turned out to be double rfe rfa mutants in which the rfe mutation (close to ilv) accounts for inability to synthesize O-specific structures and the rfa mutation accounts for defects in the synthesis of the core and, thus, for the lipopolysaccharide chemotype. These mutants demonstrate that in S. minnesota of O group L, as well as in S. typhimurium of the O group B, the rfb gene cluster close to his determines the synthesis of the O-specific side chain of the lipopolysaccharide, and the rfa genes in a cluster close to pyrE determine the synthesis of the lipopolysaccharide core. Phosphorylation of the core is determined by a rfaP gene also in this cluster and so far identified only by these S. minnesota mutants. In addition, the rfe gene(s) close to ilv is required for O-side chain synthesis in S. minnesota but not in S. typhimurium.