Staphylokinase (SAK) is a protein fromStaphy-lococcus aureusthat activates plasminogenby forminga1 : 1 complex with plasmin. Recombinant SAK has been shown in clinical trials to induce ®brin-speci®c clot lysis in patients withacutemyocardial , SAKelicits high titers of neutralizing antibodies. Biochemical and protein engineering studies have demonstrated the feasibility of generatingSAKvariantswith reducedantigenicity yet intact thrombolytic potency.