A rapid in vitro method for measuring antioxidant activity is presented, which enables the evaluation of health claims and the optimization of product development with respect to health protecting compounds. Antioxidant activity is assessed in a system in which lipid peroxidation is induced in male rat liver microsomes by ascorbic acid and FeSO(4). This method has been significantly improved by enabling the use of microtiter plates and an ELISA reader. Large numbers of samples can be analyzed with good reproducibility, which is necessary when dealing with microsomes possessing biological variability. An objective mathematical procedure has been developed to translate data obtained from the lipid peroxidation assay into a value describing the antioxidant activity. As an illustration the method has been applied to measure antioxidant activity of individual flavonoids and apple juice.