Authors studied the cytotoxic activity (CA) of natural killer cells (NK) in 16 schizophrenic patients, aged from 18 to 25 years, with the first episode of psychosis. A significant decrease in CA NK was found in the total group of patients compared to healthy controls that suggested the dysregulation of natural immunity at this stage of disease. The addition of serotonin to the cell culture in vitro in concentrations 10-9 M stimulated the NK activity while the elimination of monocytes resulted in the significant reduction of CA NK. The parallel use of two methods for assessment of CA NK (a radioactive method and an easy and safe method using multifunctional particle size analyzer Multisizer MS-4) revealed similar results.