The aim of this investigation was to measure the activity of four 4-hydroxycoumarin derivatives - three of them were described before and one was newly synthesized. The substances were ethyl 2-[(4-hydroxy-2-oxo-2H-chromen-3-yl)(4-hydroxyphenyl)methyl]-3-oxobutanoate (SS-14), 4-[1-(4-hydroxy-2-oxo-2H-chromen-3-yl)-2-(ethoxycarbonyl)-3-oxobutyl]benzoic acid (SS-17), ethyl 2-[(4-hydroxy-2-oxo-2H-chromen-3-yl)(3-nitrophenyl)methyl]-3-oxobutanoate (SS-21) and ethyl 2-[(3,4,5-trimethoxyphenyl)(4-hydroxy-2-oxo-2H-chromen-3-yl)methyl]-3-oxobutanoate (T-2). The synthesis of T-2 consists of two steps. First step was Knoevenagel reaction between 3,4,5-trimethoxybenzaldehyde and ethylacetoacetate. Ethyl 2-(3,4,5-trimethoxy)-phenylmethyleneacetoacetate was the product. Second step was Michael addition reaction between the latter product and 4-hydroxycoumarin. All the compounds were tested in vitro for antioxidant activity in hypochlorous system. The assay was based on the luminol-dependent chemiluminescence of free radicals, which decreased in the presence of 4-hydroxycoumarin derivative. Compound SS-14 (ethyl 2-[(4-hydroxy-2-oxo-2H-chromen-3-yl)(4-hydroxyphenyl)methyl]-3-oxobutanoate) expresses the best scavenger activity at the highest concentration (10(-4)mol/L).