A study of the activity of leukocyte enzymes in the blood of patients with nuclear forms of schizophrenia (52 cases) and in patients with circular schizophrenia (22 cases) depicted the following conditions. In the group of malignant schizophrenia, irrespective of the stage of the disease and in the group of circular forms there was a definite drop in the activity of cytochromoxidase, succinatedehydrogenase and MAO, while as the activity of the ATP-ase and peroxidase was increased. Supplementary animal experiments in vitro and in vivo made it possible to assume that these changes are due to the so-called serum factor.