Erythrocytes from patients with chronic hemolytic variants of glucose-6-phosphate dehydrogenase (G-6-PD) deficiency have structural membrane protein abnormalities accompanied by decreased cell membrane deformability which the authors postulate represent the consequences of oxidant-induced membrane injury. To evaluate the pathophysiologic significance of oxidant-induced membrane injury, they studied the in vitro and in vivo effects of the thiol-oxidizing agent, diamide, on dog erythrocytes. In vitro incubation of dog erythrocytes with 0.4 mM diamide in Tris-buffered saline for 90 min at 37°C resulted in depletion of GSH, formation of membrane polypeptide aggregates (440,000 and >50,000,000 daltons) and decreased cell micropipette deformability, abnormalities similar to those observed in the erythrocytes of patients with chronic hemolytic variants of G-6-PD deficiency. In addition, diamide-incubated cells had increased viscosity and increased membrane specific gravity, but no change in ATP. Reinjection of 51Cr-labeled, diamide-incubated cells was followed by markedly shortened in vivo survival and splenic sequestration. Further incubation of diamide-incubated cells in 4 mM dithiothreitol reversed the membrane polypeptide aggregates, normalized micropeptide deformability, decreased cell viscosity, prolonged in vivo survival, and decreased splenic sequestration. These studies demonstrate that diamide induces a partially reversible erythrocyte lesion which is a useful model of oxidant-induced membrane injury. They suggest that oxidant-induced erythrocyte membrane injury plays an important role in the pathophysiology of chronic hemolysis which accompanies some G-6-PD variants.