The fluorescent polyunsaturated parinaric acid incorporated in LDL particles is highly sensitive to the concentration of peroxyl radicals in the aqueous medium, undergoing rapidly oxidative degradation, as detected by a quenching of fluorescence, without delay after radical generation in solution. Ascorbate, cysteine, and urate suppress the parinaric acid fluorescence decay promoted by peroxyl radicals generated at a constant rate (thermal decomposition of 2,2'-azo-bis(2-amidino-propane hydrochloride)) in a concentration-dependent manner. The chain-breaking efficiencies of these antioxidants are evaluated from the time interval (inhibition period) of parinaric acid protection from oxidative degradation. The results correlate with the inhibition periods of LDL oxidation as monitored by O2 consumption. Therefore, the sensitive and simple parinaric acid assay can be used as a semiquantitative screening test for the detection of potentially important water-soluble chain-breaking antioxidants. Conversely to O2 consumption, the absence of any initial lag phase of probe degradation attests to the sensitivity of the assay. An improved methodology based on second-derivative spectroscopy to follow the formation of conjugated diene isomers directly in the preparation without the need for lipid extraction also confirms the sensitivity of this assay. To assess the usefulness of parinaric acid assay, strong chain-breaking activities of caffeic and chlorogenic acids are reported.