ADP-ribosylhydrolases catalyze the release of ADP-ribose from ADP-ribosylated proteins via hydrolysis of the glycosidic bond between ADP-ribose and a specific amino-acid residue in a target protein. Human ADP-ribosylhydrolase 3, consisting of 347 amino-acid residues, has been cloned and heterologously expressed in Escherichia coli, purified and crystallized in two different space groups. Preliminary X-ray diffraction studies yielded excellent diffraction data to a resolution of 1.6 A.