Partial deficiency of 6-phosphogluconolactonase (EC 3.1.1.31) of the erythrocytes was discovered as an autosomal dominant disorder. Hemolytic anemia occurred in an individual who had inherited both the gene for 6-phosphogluconolactonase deficiency and that for deficiency of a nonhemolytic variant of glucose-6-phosphate dehydrogenase (EC 1.1.1.49). It is proposed that the interaction of this hereditary erythrocyte abnormality with glucose-6-phosphate dehydrogenase deficiency may explain hemolysis in some other patients who have inherited polymorphic variants of glucose-6-phosphate dehydrogenase.