Repair of heteroduplex DNA containing an A/G mismatch in a mutL background requires the Escherichia coli mutY gene function. The mutY-dependent in vitro repair of A/G mismatches is accompanied by repair DNA synthesis on the DNA strand bearing mispaired adenines. The size of the mutY-dependent repair tract was measured by the specific incorporation of alpha-[32P]dCTP into different restriction fragments of the repaired DNA. The repair tract is shorter than 12 nucleotides and longer than 5 nucleotides and is localized to the 3' side of the mismatched adenine. This repair synthesis is carried out by DNA polymerase I.