The metabolism of the three branched-chain amino acids has been investigated in vitro, using the peripheral leukocyte. The normal leukocyte can transaminate and decarboxylate the three amino acids. These functions are demonstrable at birth. Five cases of maple syrup urine disease (branched-chain ketoaciduria) were studied. The peripheral leukocyte could transaminate the three amino acids, but decarboxylation was greatly reduced or absent. This confirms the site of metabolic block in maple syrup urine disease, and suggests an early and specific approach to diagnosis. Oxidative-decarboxylation of the branched-chain ketoacids involves an enzyme common to all three ketoacids.