The presence of creatine, guanidinopropionic acid (GPA) and guanidinobutyric acid (GBA) was demonstrated in red blood cells from uremic patients; they were found only in trace amounts in red blood cells of normal controls. The levels of creatine, GPA and GBA in the red cell did not change during dialysis in contrast to the simultaneous decrease in plasma level. Both creatine and GPA inhibited glucose-6-phosphate dehydrogenase (G6PD) in vitro in physiological concentration, while creatine also activated erythrocyte transketolase (ETK). These effects are consistent with the low red cell G6PD level and high ETK activity that were observed in our uremic patients. The unchanging levels of creatine and GPA in the red cell despite hemodialysis may explain the continuing autohemolysis in otherwise adequately hemodialyzed end-stage renal failure patients.