DNA deoxyribophosphodiesterase (dRpase) of E. coli catalyzes the release of deoxyribose-phosphate moieties following the cleavage of DNA at an apurinic/apyrimidinic (AP) site by either an AP endonuclease or AP lyase. Exonuclease I is a single-strand specific DNA nuclease which affects the expression of recombination and repair pathways in E. coli. We show here that a major dRpase activity in E. coli is associated with the exonuclease I protein. Highly purified exonuclease I isolated from an over-producing stain contains high levels of dRpase activity; it catalyzes the release of deoxyribose-5-phosphate from an AP site incised with endonuclease IV of E. coli and the release of 4-hydroxy-2-pentenal-5-phosphate from an AP site incised by the AP lyase activity of endonuclease III of E. coli. A strain containing a deletion of the sbcB gene showed little dRpase activity; the activity could be restored by transformation of the strain with a plasmid containing the sbcB gene. The dRpase activity isolated from an overproducing stain was increased 70-fold as compared to a normal sbcB+ strain (AB3027). These results suggest that the dRpase activity may be important in pathways for both DNA repair and recombination.