UV damage endonuclease (UVDE) initiates a novel form of excision repair by introducing a nick imme-diately 5" to UV-induced cyclobutane pyrimidine dimers or 6-4 photoproducts. Here, we report that apurinic/apyrimidinic (AP) sites are also nicked by Neurospora crassa and Schizosaccharomyces pombe UVDE. UVDE introduces a nick immediately 5" to the AP site leaving a 3"-OH and a 5"-phosphate AP. Apyrimidinic sites are more effectively nicked by UVDE than apurinic sites. UVDE also possesses 3"-repair activities for AP sites nicked by AP lyase and for 3"-phosphoglycolate produced by bleomycin. The Uvde gene introduced into Escherichia coli cells lacking two types of AP endonuclease, Exo III and Endo IV, gave the host cells resistance to methylmethane sulfonate and t-butyl hydroperoxide. We identified two AP endonuclease activities in S.pombe cell extracts. Besides cyclobutane pyrimidine dimers and 6-4 photoproducts, N. crassa UVDE also nicks Dewar photoproducts. Thus, UVDE is able to repair both of the major forms of DNA damage in living organisms: UV-induced DNA lesions and AP sites.