2,3-Dihydroxybenzoic acid (2,3-DHB) inhibits the second wave of platelet aggregation release of serotonin, and malonaldehyde production. The effect is concentration dependent and reversible in vitro and in vivo. 50% inhibition of platelet aggregation induced in vitro with 70 micron adrenaline was obtained with 1.5 mM 2,3-DHB. Malonaldehyde production was inhibited by 50% with 0.4 mM 2,3-DHB. Serotonin release was 50% inhibited by 50% with 0.08 mm 2,3-DHB. Neither desferrioxamine nor various benzoic acid derivatives with iron-chelating properties showed the effect. Only benzoic acid derivatives with the potential to terminate free radical reactions through the formation of an ortho- or paraquinone structure inhibit the second wave of platelet aggregation. These studies support the concept that the second wave of platelet aggregation is mediated through free radical reactions.