The molecular nature of the glycine cleavage system was investigated in 16 patients with nonketotic hyperglycinemia (NKH). The overall activity of the glycine cleavage system was found to be decreased in all of the liver and brain tissue studied. It was undetectable or extremely low in the neonatal type of NKH, whereas there was some residual activity in the infantile type of NKH. Thus the clinical phenotypes do seem to relate to the degree of the defect in the glycine cleavage system. In the neonatal type, a specific defect in P protein was found in 9 cases and a specific defect in T protein in 2 cases. In the infantile type, a partial defect in T protein was found in 2 cases. Differential diagnosis between NKH and ketotic hyperglycinemia is described. A feasibility of prenatal diagnosis of NKH by chorionic villus biopsy is provided.