Objective. To compare glutathione reductase (GR) and glutathione-S-transferase (GST) enzymatic activities in blood cells (erythrocytes and platelets) of patients with schizophrenia spectrum disorders and in the control group and to search for correlations of these biochemical parameters with clinical psychiatric assessments of the patient. Material and methods. The study included patients (97 men) with schizophrenia spectrum disorder (schizophrenia and schizoaffective disorders) in an acute state of exacerbation of psychotic symptoms and 33 men without mental pathology. Symptom severity was measured with the PANSS before and after antipsychotic therapy. GR and GST activities were determined spectrophotometrically. Results. There were no significant between-group differences in the activities of erythrocyte GR and GST. In platelets, the GR activity was lower in all patients’ groups than in controls, whereas the GST activity in patients with schizophrenia relapses and in patients with schizoaffective disorder (SD) was lower than in controls (p<0.05) both before and after treatment. Differences between subgroups of first-episode patients (schizophrenia and SD) and patients with schizophrenia relapses were found not only in the levels of erythrocyte and platelet GR and GST activities, but also in the changes of these enzymatic activity levels under antipsychotic treatment, as well as in links binding these enzymatic activities and PANSS scores. Conclusion. The decreased level of GR and GST, the glutathione-dependent enzymes, contributes to the reduction of antioxidant defense in schizophrenia spectrum disorders. The correlations linking the basal levels of GR and GST activities with the results of clinical assessments after treatment allow us to consider these parameters as potential biomarkers for predicting treatment response.