Oxidant stress leads to covalent oxidative modification of several plasma proteins, chief among which is fibrinogen. Aspirin can nonenzymatically acetylate fibrinogen's lysine residues, the functional groups most susceptible to oxidative modification. Because oxidation of fibrinogen may occur in the atheromatous environment, we studied the effects of oxidative modification on fibrinogen function and the consequences of acetylation by aspirin on fibrinogen's susceptibility to oxidation and functional properties. We exposed fibrinogen to Fe3+ ascorbate for 1 hour and showed that the carbonyl/protein molar ratio increased from 0.71 ± 0.18 to 2.86 ± 0.50 mol carbonyl/mol protein (P < 0.02) with an accompanying reduction in the α-helical content of the protein from 34% to 29%. Exposure of fibrinogen to aspirin led to acetylation of lysine residues and inhibition of oxidation. Oxidized fibrinogen was more readily able to form fibrin, and acetylation prevented this enhancement of clot formation. Oxidized fibrinogen also supported platelet aggregation better than did native, unoxidized fibrinogen, and acetylation of fibrinogen prior to oxidation prevented the enhanced platelet aggregation. Oxidized fibrinogen was less effective in stimulating plasminogen activation by tissue-type plasminogen activator (t-PA), with a catalytic efficiency that was reduced by 88% compared with native, unoxidized fibrinogen; acetylated fibrinogen, by contrast, enhanced plasminogen activation by t-PA with a catalytic efficiency that was increased by 18% compared with native, unoxidized fibrinogen (P < 0.05) and was increased by 51% compared with oxidized fibrinogen(P < 0.05). Acetylation prevented the reduction in catalytic efficiency induced by oxidation. These data show that oxidized fibrinogen manifests prothrombotic effects that can be prevented by acetylation and suggest that inhibition of fibrinogen oxidation may be an additional antithrombotic benefit of aspirin therapy.