The synthesis and rapid turnover of ADP-ribose polymers is an immediate cellular response to DNA damage. We report here the isolation and characterization of cDNA encoding poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for polymer turnover. PARG was isolated from bovine thymus, yielding a protein of approximately 59 kDa. Based on the sequence of oligopeptides derived from the enzyme, polymerase chain reaction products and partial cDNA clones were isolated and used to construct a putative full-length cDNA. The cDNA of approximately 4.1 kilobase pairs predicted expression of a protein of approximately 111 kDa, nearly twice the size of the isolated protein. A single transcript of approximately 4. 3 kilobase pairs was detected in bovine kidney poly(A)+ RNA, consistent with expression of a protein of 111 kDa. Expression of the cDNA in Escherichia coli resulted in an enzymatically active protein of 111 kDa and an active fragment of 59 kDa. Analysis of restriction endonuclease fragments from bovine DNA by Southern hybridization indicated that PARG is encoded by a single copy gene. Taken together, the results indicate that previous reports of multiple PARGs can be explained by proteolysis of an 111-kDa enzyme. The deduced amino acid sequence of the bovine PARG shares little or no homology with other known proteins. However, it contains a putative bipartite nuclear location signal as would be predicted for a nuclear protein. The availability of cDNA clones for PARG should facilitate structure-function studies of the enzyme and its involvement in cellular responses to genomic damage.