Studies of red cell metabolism, erythropoeitin concentration, iron and folate status were made in 48 children with protein-energy malnutrition in Johannesburg (altitude 1800 m). Biochemical evidence of iron deficiency was presented in 26% cases on admission and developed in 90% during recovery. Biochemical evidence of folate deficiency was present in 14% of cases on admission and resolved on dietary therapy alone. Serum erythropoeitin was increased on admission and remained elevated during recovery. There was no relationship between serum erythropoeitin and Hb concentrations. Key enzymes in the red cell glycolytic and hexose monophosphate pathways and red cell membrane showed increased activity. Red cell adenosine triphosphate concentration was increased and unstable. Red cell potassium was decreased and, in the fatal cases, red cell sodium was increased. The possible significance and practical implications of these findings are discussed.