ADP-ribosyltransferases catalyze the transfer of the ADP-ribose moiety from NAD(+) onto proteins and other targets. These enzymes have been found in prokaryotes and in vertebrates; a eukaryotic enzyme structure is not yet known. The enzyme from Rattus norvegicus was expressed in the Escherichia coli periplasm at a level of about 0.2 mg per litre of culture, purified and crystallized. Native data sets were collected to 2.0 A resolution. A self-rotation function revealed a local twofold axis in crystal form A and a Patterson function showed a translational relationship in form B. Form C contains only one molecule in the asymmetric unit.