Abstract A neutral, membrane-bound, phosphatase activity was characterized in normal red blood cells, using p-nitrophenylphosphate as substrate. Its specific activity was 1.59 nmol mg −1 min −1. The kinetics were of the Michaelis type: K M,app = 2.5 × 10 −3 M. It was stimulated by K + and inhibited by ouabain, a behaviour reminiscent of (Na + + K +)-ATPase. In 10 patients with homozygous sickle cell disease and in 11 patients with unidentified congenital hemolytic anemias, the specific activity was significantly increased. In general, the phosphatase retained Michaelis-Menten kinetics. However, in four patients from the same family with an unidentified hemolytic anemia, the kinetics yielded a biphasic curve instead of a rectangular hyperbola, a change consistent with the existence of an inhibition by substrate excess. From detailed analysis of the curve, the apparent inhibitor constant for pNPP was determined: K i,app approx. 2.5 × 10 −2 M. This novel abnormality of the red cell membrane might be the distinctive feature of a given type of congenital hemolytic anemia.