A transducing phage carrying some of the genes (men) defining the early stages of menaquinone biosynthesis was isolated from a pool of recombinant lambda phages that had been constructed from R.HindIII digests of E. coli DNA and the corresponding insertion vector. The lesions of menB and menC mutants were complemented by the phage but menD mutants were transduced either at low frequencies or not at all. This indicates that the transducing phage contains functional menB and menC genes but that only part of the menD gene had been cloned. The phage (lambda G68) was accordingly designated lambda menCB(D). Studies with the transducing phage enabled earlier mapping data (Guest 1979) to be reinterpreted in favour of the gene order nalA....menC..menB..MenD....purF. Restriction analyses established the presence of a bacterial DNA fragment (11.5 kb) linked by a R.HindIII target to the right arm of the lambda genome but fused to the left arm of the vector. Hybridization studies confirmed that the cloned DNA was derived from a larger R.HindIII fragment (21 kb). A physical map of the men region was constructed and some flanking and overlapping fragments were identified.