Paroxysmal nocturnal hemoglobinuria (PNH) is a myelodysplastic disease characterized by erythrocytes that show abnormally increased sensitivity to complement-mediated lysis. Complement-sensitive PNH erythrocyte membranes have previously been shown to lack acetylcholinesterase (AchE) activity, but the molecular basis of this deficiency has been unclear. We have used monoclonal antibodies to four different epitopes on the AchE molecule to show that abnormal PNH erythrocytes failed to bind these antibodies. Moreover, abnormal PNH erythrocytes contained no protein immunoprecipitable by these antibodies, while normal complement-insensitive erythrocytes from PNH patients showed normal amounts of immunoprecipitable AchE which had normal electrophoretic mobility. These data suggest that abnormal PNH erythrocytes lack AchE enzyme activity due to the absence of the AchE molecule from the cell membrane.