Salicylate is known to uncouple mitochondrial oxidative phosphorylation. Since the viability of pyruvate-kinase-deficient reticulocytes depends on ATP generated by mitochondrial metabolism, this study examined the effects of salicylate on erythrocytes deficient in pyruvate kinase. When deficient erythrocytes from patients with severe hemolysis were incubated with salicylate (2 to 30 mg per deciliter), there was a marked decrease (25 to 75 percent) in ATP. In addition, this drug-induced ATP depletion produced cell potassium and water loss, and the normal oxidant responsiveness of the hexose-monophosphate shunt was blunted. Since these cellular abnormalities are associated with accelerated hemolysis in vivo, the data suggest that aspirin therapy may aggravate hemolysis in patients with pyruvate kinase deficiency whose erythrocyte manifest sensitivity to salicylate in vitro.