A plasma factor that inhibits red cell hexose monophosphate (HMP) shunt metabolism and thereby shortens red cell survival was found to accumulate in roughly half of uremic patients. The factor is efficiently removed by frequent hemodialysis with purified water in the bath solutions. However, contaminants in tap water hemodialysis baths potentiate the red cell defect, and hemolysis may strikingly worsen in patients so dialyzed. As in patients with deficient red cell glucose 6 phosphate dehydrogenase, and thus deficient HMP shunt metabolism, explosive Heinz body hemolytic anemia may occur in affected uremic patients given oxidant drugs, such as sulfonamides, antimalarials, and the like. The authors urge that uremic patients be screened for red cell HMP shunt deficiency before administration of sulfonamides or other oxidant compounds and that affected patients be dialyzed only against purified water baths.