The effects of coumarin and its derivatives on rat platelet lipoxygenase and cyclooxygenase activities were studied. Esculetin (6,7-dihydroxycoumarin) was found to inhibit the lipoxygenase more strongly than the cyclooxygenase; its concentration for 50% inhibition (IC50) was 0.65 microM for platelet lipoxygenase and 0.45 mM for platelet cyclooxygenase. Esculin (the 6-glucoside of esculetin) and umbelliferone (7-hydroxy-coumarin) also selectively inhibited the lipoxygenase, though less strongly (IC50 = 290 and 500 microM, respectively). 4-Hydroxycoumarin and coumarin had no inhibitory effect on either enzyme at concentrations up to 1 mM. The mechanism of the lipoxygenase inhibition by esculetin was non-competitive. Other antioxidants (hydroquinone, gallic acid and ascorbic acid) were less inhibitory to both enzymes and showed little selectivity.