Abstract A specialized A transducing phage carrying the mutH gene and several deletion derivatives of this phage were characterised by restriction enzyme analysis. This analysis localized the mutH gene to a small region of bacterial DNA on the transducing phage and facilitated the subsequent cloning of this gene into the multicopy plasmid pBR322. The mutH gene is contained entirely on a 1.5-kb HinddIII fragment as judged by the ability of plasmids carrying this fragment to complement mutH alleles on the bacterial chromosome. Using recombinant plasmids containing the 1.5-kb HindIII fragment, we identified an M M r 25000 protein as the product of the mutH gene in an in vitro transcription translation system as well as in maxicells. Various deletion derivatives of the mutH-containing plasmids that exhibit a Mut phenotype also have lost the M r 25000 protein.