Red blood cells (RBC) of children with terminal renal insufficiency show a reduction of the apparent 51Cr RBC half-life (16.5 ± 5.5 days). The enzymatic activities of phosphofructokinase (PFK) and glucose 6-phosphate dehydrogenase (G6PD) in the RBC of these patients do not differ from those in healthy persons. The RBC of uraemic and healthy children were separated by density gradient centrifugation in dextran medium according to maturity and age. RET count, mean cellular Hb concentration (MCHC), PFK and G6PD activities were determined in the separated cell populations. MCHC increases with increasing cell density, while the number of RET decreases. There is a distinct decrease in the G6PD activity of low-density cells compared to high-density cells. The extent of the decrease in enzymatic activity is about the same for healthy persons as for uraemic patients. A more rapid decrease in G6PD activity per unit of time can be assumed when the enzymatic activity is plotted against the distinctly shortened life span of the RBC of the uraemic children. There is a possible relationship between G6PD activity in old RBC and the premature destruction of these cells. PFK activity apparently does not decrease with increasing cell age.