A PC-controlled sequential injection analysis (SIA) system equipped with a spectrophotometric diode-array detector is used for rapid monitoring and evaluation of antioxidation/radical scavenging activity of biological samples. The automated method is based on the known reaction of stable 2,2'-diphenyl-1-picrylhydrazyl radical (DPPH) with antioxidants in organic or aqueous-organic media resulting in bleaching of DPPH due to its "quenching" by the interaction with the analytes. The decrease of the absorbance of DPPH (compared to blank experiment carried out with water-ethanol 1:1 instead of the test solution) measured at 525 nm is related to concentration of an antioxidant in the test solution. With the optimised SIA procedure it is possible to detect down to micromolar concentrations of model antioxidants such as ascorbic acid, caffeic acid, (+)-catechin, (-)-epicatechin and rutin and to evaluate the concentration of these antioxidants in the micromolar to millimolar range. The sample throughput is 45 h(-1). Thanks to its rapidity and sensitivity, the proposed SIA method is suitable for performing routine screening tests for the presence of various antioxidants in large series of lyophilised herbal or mushroom extracts (the amount of sample needed for the analysis is several milligrams).