Excision of cyclobutyl dipyrimidines from, and accumulation of strand interruptions in DNA of different strains of E.coli K12 were determined during liquid holding recovery after uv irradiation. The extent of Pyr <> Pyr excision was the same (20 to 25%) for both a polA mutant (E.coli P3478) and its parental wild type strain (E.coli W3110); however, single strand interruptions accumulated during liquid holding of polA cells, but not in the parental strain. In contrast, excision was greatly reduced in a mutant (KMBL 1789) which is defective in the 5' ..-->.. 3' exonucleolytic function of DNA polymerase I. These data suggest that excision and resynthesis during liquid holding are carried out primarily, if not entirely, by DNA polymerase I. It is further concluded that excision alone is both a necessary and sufficient condition to elicit liquid holding recovery, and that this excision requires a functional polymerase I 5' ..-->.. 3' exonuclease.