This report describes an assay with the capability of detecting antioxidant activity of plasma and of purified proteins. The method is based on the enzymatic oxidation of plasma elastase inhibitory capacity of enzyme(s) in extracts of human lysosomes in the presence of hydrogen peroxide, chloride, and magnesium ions. Using this method we measured the antioxidant activity of the plasma of 59 subjects. We detected a strong relationship between a deficiency in the antioxidant activity of plasma and the presence of a family history of lung disease (p less than 0.005), and an abnormal mean FEV1/FVC ratio (p less than 0.005).