Kynureninase activity was investigated in the livers biopsied from the patients with B6 dependent xanthurenic aciduria. The activity was found to be markedly reduced but it was restored to a considerable extent by the addition of' an excess of pyridoxal phosphate to the assay system. This suggests that the basic defect in B_6 dependency is an inability of apoenzyme to combine with the coenzyme. Glycine cleavage system was investigated in the livers and brains autopsied from patients with nonketotic hyperglycinemia. The overall activity of the glycine cleavage system was found to be extremely low in both the liver and brain. A-nalyses of four protein components involved in glycine cleavage system revealed that nonketotic hyperglycinemia is caused by a specific defect in either pyridoxal phosphate-dependent protein or tetrahydrofolate-requiring protein. Key Words: B6 dependency, familial xanthurenic aciduria, hyperglycinemia, glycine cleavage reaction