The levels of erythrocyte glutathione and the activities of its metabolizing enzymes--glutathione peroxidase (GSH-Px), glutathione S-transferase (GST) and glutathione reductase (GR)--were measured in four species of primates: human, rhesus monkey, common marmoset and common tree shrew. 2. There were marked differences in GSH-Px and GST activities among the primates, while GR activity and glutathione level were much less variable.