When intact rabbit erythrocytes were treated with pancreatic lipase, the membrane-bound NAD nucleosidase (NADase) was partially solubilized without marked destruction of the cellular stucture, suggesting that the entire molecule of the rabbit erythrocyte NADase is located on the outer portion of the cell membrane. The lipase-solubilized NADase from rabbit erythrocyte membrance was partially purified 26-fold, and its properties were characterized. Among other properties the most characteristic one was that the rabbit erythrocyte NADase was activated by urea in low concentrations, whereas NADases from other sources were not affected. Urea (0.2 M) caused three-fold increases in both the Michaelis constant for NAD and the maximal velocity of NADase, thus making the reciprocal plot in the presence of urea parallel to the plot in the absence of urea. The competitive inhibitors such as NADP and nicotinamide mononucleotide did not eliminate the activation effect of urea, while nicotinamide and 3-acetylpyridine known as noncompetitive inhibitor eliminated the urea effect. These results strongly imply that the activation of NADase by urea in low concentrations is of uncompetitive nature, suggesting that the action of urea should enhance the rate of the enzyme-substrate complex breakdown involved in the liberation of nicotinamide and not earlier step concerned with the formation of the enzyme-substrate complex.