Objectives: The aim of the present study was to examine the susceptibility of glutathione (GSH) and glutathione related antioxidant enzymes to oxidation in type 2 diabetic patients with and without glycemic control. Design and methods: Erythrocyte glutathione level and activities of glutathione peroxidase (G-Px), glutathione reductase (G-Red) and glutathione S-transferase (GST) in controls, well controlled and poorly controlled type 2 diabetics were measured by spectrophotometric assays before and after the incubation in vitro with H2O2. Results: GSH level, G-Px and G-Red activities decreased but GST activity increased in the erythrocytes from all the groups after the incubation with H2O2. Percentage of decrease in GSH was independent from glycemic control, whereas the percentage of decreases in G-Px and G-Red was related to glycemic control. The percentage of increase in GST was found to be independent from diabetes Conclusions: GSH and GSH-related antioxidant enzymes in human erythrocytes are susceptible to oxidation, particularly, G-Px and G-Red which were found to be more susceptible to oxidation in erythrocytes from poorly controlled type 2 diabetic patients. © 2002 The Canadian Society of Clinical Chemists. All rights reserved.