Antisera inhibitory to NAD nucleosidase (NADase) of rabbit erythrocyte membrane (ghost) have been produced in roosters by intravenous injection of whole rabbit erythrocytes or the erythrocyte ghost preparation as antigens. As in a typical precipitin test of antigen-antibody reaction, the maximal inhibition of hte ghost NADase by antisera to rabbit erythrocytes occurred at an optimal antigen : antibody ratio, and the inhibition decreased in an excess of antigen or antibody. The inhibition was time-dependent, maximal inhibition being attained by 6 hours` incubation at 37°. Aggregation and precipitation occurred within a few minutes during incubation of the ghost or whole erythrocyte suspension with antisera. The precipitates, however, were enzymatically active. The degree of enzyme inhibition was roughly paralleled to the extent of precipitation. When chicken antisera were heat-treated at 56° for 20 minutes, the NADase inhibition decreased markedly. The NADases of rabbit spleen, liver and heart were inhibited in varying degrees by antirabbit erythrocyte chicken sera, while the enzymes from rabbit lung and kidney were not inhibited at all. Considerable cross reaction was observed between the ghost NADase of swine and antisera both to rabbit erythrocytes and to erythrocyte ghost preparation. No significant reaction occurred toward sheep and calf ghost preparations. These results suggest that the inhibition of the homologous ghost NADase by antirabbit erythrocyte or ghost chicken sera might be due to the formation of precipitate which is caused by combination of heterogenous antibodies with various antigenic determinants of the ghost preparation other than with the active site of NADase.