Enzymatic activity of NADH-monodehydroascorbate reductase could be observed in red blood cell membranes. This activity was latent in right side out as well as inside out vesicles. Apart from this latency addition of certain detergents led to activation of the enzyme also in open membrane preparations. The enzyme was inhibited by metal chelators, and displayed a very low apparent Michaelis constant. Monodehydroascorbate is a candidate for the natural electron acceptor of the transmembrane NADH-oxido-reductase. The activation by detergent may be due to enhancement of lipid fluidity or to exposure of a lipophilic substrate binding site.