Middle molecule (MM) fractions were isolated from the erythrocytes of patients in chronic renal failure on conservative treatment, and healthy subjects and fractionated on Sephadex G 15. Most MM fractions were found in markedly higher concentrations in erythrocytes than in plasma. A model inhibitor of gluconeogenesis found previously in plasma and urine of the uraemic subjects was found also in the erythrocytes. Repeated washing of the erythrocytes with isotonic solution of NaCl did not wash out the inhibitor of gluconeogenesis from erythrocytes.