The concentrations of circulating immune complexes (IC) were determined in the blood serum of 105 schizophrenics, 22 chronic alcoholics and 26 patients with CNS organic lesions. To assess antigen-antibody complexes, the authors used the method based on the principle of selective IC precipitation by polyethylene glycol-6000. It was established that the groups of schizophrenic patients and patients with organic CNS impairments included a lot of individuals with elevated levels of the immune complexes. Sex differences were disclosed with regard to this indicator, which was higher in women than in men, while in mentally ill women, it was highest than in healthy ones. The levels of immune complexes in patients with schizophrenia tended to rise with the increasing duration of the disease.