Phosphoglycerate kinase deficiency is a rare, x-linked glycolytic defect that, when severe, can be associated with hemolytic anemia, rhabdomyolysis, or neurological disorders. We report here a new phosphoglycerate kinase variant discovered in a boy with severe hemolytic anemia but no evidence of neuromuscular disease or developmental delay. The biochemical properties of the variant enzyme (greatly increased kmATP and km3-phosphoglycerate; normal pH optimum, electrophoretic mobility, and substrate specificity; resistance to heat inactivation) establish its uniqueness. Separation of light and dense red cells by centrifugation showed no greater loss of phosphoglycerate kinase activity in dense ("old") variant cells than in normal cells. We postulate that the striking stability of the variant enzyme allows cells capable of protein synthesis to accumulate sufficient enzyme to limit neuromuscular sequelae.