A mutant of Escherichia coli K-12 carrying a single mutation, designated rfaD, which affects the synthesis of the aldoheptose, L-glycero-D-mannoheptose, has been isolated. Phenotypically this mutant is novobiocin-hypersensitive with altered lipopolysaccharide (LPS) core biosynthesis. It was observed that the LPS of the rfaD mutant contained the aldoheptose D-glycero-D-mannoheptose rather than the normal mutant L-glycero-D-mannoheptose. It is postulated that L-glycero-D-mannoheptose is formed from D-glycero-D-mannoheptose via a racemization. A crude assay to measure the epimerase activity has been developed. Hybrid colicin El plasmids capable of correcting the rfaD phenotype have been selected, and these increase the epimerase activity and the number of completed LPS core structures relative to the wild type. This represents the initial step in understanding the biochemistry of aldoheptose biosynthesis and provides a system for studying its regulation. I selected from the Clark and Carbon colony bank, hybrid ColEl plasmids which corrected the rfaD phenotype and conferred ColEl resistance by F-factor--mediated transfer. This was further demonstrated by transforming experiments using hsd rfaD strains. We are now characterizing the hybrid ColEl plasmids, cloning and restriction mapping the rfaD gene fragment.