AN ACUTE transitory cessation of red-blood-cell production in hereditary spherocytosis was reported by Owren,1 who applied the term "aplastic crisis" to the precipitous drop in red-blood-cell count and hemoglobin levels noted in his patients. Similar observations have also been made by Dameshek and Bloom,2 Horne and co-workers,3 and others4 on persons with familial hemolytic jaundice. Dameshek and Bloom2 suggested that the aplastic crisis was secondary to increased splenic activity, which produced a maturation arrest of the red-cell apparatus at the pronormoblast level. Recently, Singer and associates5 described the development of an aplastic crisis in a patient with sickle-cell anemia. The pathophysiologic mechanisms responsible for the development of such an event in the hemolytic syndromes have been extensively discussed by both Owren1 and Singer and others.5 No evidence of hypersplenism, either in inhibiting the marrow or in causing increased hemolysis of the red