Alteration of redox balance in the serum of patients with systemic lupus erythematosus (SLE) and systemic vasculitis (SV) was investigated. Excess in oxidative processes has been measured through concentration of lipid peroxides which was found to increase by 26% in SLE and 32% in SV. Antioxidant protection capacity against this oxidative aggression has been assessed both by determining the level of activity of the enzymes participating in this process (superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase GSH-Px) and by determining the total antioxidant serum activity. The results have shown that, within antioxidant protection of the body against oxidative stress, glutathione peroxidase plays the most important role and its activity is significantly affected by the great concentration of lipidic peroxides. We have also shown that there is positive correlation (r = 0.91) between the level of lipidic peroxides and the extent to which the tissue is affected. The latter is assessed by studying the serum activity of lactate dehydrogenase. Therefore, these two biologic parameters are shown to be very useful when the study of the development of the diseases is undertaken.