Erythrocyte lipid peroxidation, oxidative hemolysis, erythrocyte antioxidant enzymes, viz. superoxide dismutase, glutathione reductase, glutathione peroxidase, catalase and plasma antioxidants, viz. vitamin A, vitamin E, vitamin C and ceruloplasmin have been determined by spectrophotometric methods in 15 patients with Parkinson's disease (PD) and in 50 controls. Lipid peroxidation, oxidative hemolysis and plasma ceruloplasmin were significantly higher in PD patients as compared to normals. Erythrocyte antioxidants in PD patients were not significantly different from the controls. However, plasma vitamin C in PD patients was significantly lower than the controls. It is concluded that these patients are under oxidative stress which points to a possible involvement of free radicals in PD.