R adioisotope studies of bilirubin turnover, ferrokinetics, and red cell survival (⁵¹Cr) in a patient with erythrocyte PK deficiency have provided evidence for prompt reticulocyte sequestration and destruction by the reticuloendothelial system. More mature erythrocytes appeared to survive well despite their deficiency of PK. PK-deficient reticulocytes, dependent upon oxidative phosphorylation for ATP production, are exquisitely sensitive to cyanide- or nitrogen-induced mitochondrial inhibition. If oxidative phosphorylation is unavailable, ATP levels decline rapidly, producing alterations in the cell membrane which allow massive losses of potassium and water. The result is a shrunken, spiculated, viscous cell whose rheologic properties would favor its sequestration by the reticuloendothelial system. Those reticulocytes with particularly low levels of PK exhibit very low glycolytic rates and thus are uniquely reliant upon oxidative phosphorylation. Other reticulocytes, better endowed with PK activity, can meet the increased ATP requirements of young erythrocytes. Upon reaching maturity, such cells have diminished ATP needs and can, therefore, survive despite their enzyme deficiency.