Serum glutamic oxalacetic transaminase (G-OT), glutamic-pyruvic transaminase (G-PT), and isocitric dehydrogenase (ICD) have been estimated in six groups of subjects, namely, normal British adults, normal Nigerian adults, undernourished Nigerian adults and children, Nigerian children with florid protein malnutrition (kwashiorkor), and cases of protein malnutrition after treatment. The normal range in Nigerian adults was apparently slightly lower than in European adults. A marked increase in serum enzyme activity was found in florid malnutrition, and a slight rise in undernutrition. In all cases serum G-PT showed lesser changes than G-OT and ICD. The causes of these abnormalities are discussed. The increased serum enzyme activity is thought to be derived primarily from the liver and possibly also from voluntary muscle. Estimation of serum enzymes is recommended in the investigation of protein malnutrition.