Authors studied the level of cytotoxic activity of natural killer lymphocytes (NKCA) and the effect of monocytes, serotonin, rate of serotonin reuptake by lymphocytes and psychotropic therapy on NKCA levels in 59 male patients, aged 18-30 years, with the first episode of attack-like progressive schizophrenia (33 patients) and schizoaffective psychosis (26 patients). All patients were examined at baseline, 4 and 8 weeks after the beginning of treatment with haloperidol and clozapine. Before the treatment, the decrease of NKCA was found in a half of patients compared to controls. In other patients, the NKCA levels were high and did not differ from those in controls. The treatment resistance was estimated as 70,6% in patients with schizophrenia with initially low NKCA levels and 30,8% in patients with schizoaffective psychosis with initially low NKCA levels. During the treatment, the initially high NKCA level decreased while the initially low level increased but remained lower compared to controls. These changes suggest the active reaction from the immune system of the patient to treatment with neuroleptics. The changes of NKCA values during the treatment were reciprocally related to the maximal rate of serotonin reuptake by lymphocytes. Serotonin added to the cell culture in vitro normalized the NKCA level in cultures with- and without monocytes. This effect was revealed in both pathologies only in responders, regardless of the presence or absence monocytes, that may be explained by the presence of active serotonin receptors on the NK cell surface in these patients.