Tissue-type plasminogen activator produced by recombinant DNA technology, has been established as an important thrombolytic agent in the treatment of acute myocardial infarction. New approaches to increase the effectiveness of this agent, including rapid high dose administration are being investigated. Several novel protein engineered variant forms of plasminogen activators have been produced that have increased thrombolytic potency in animal models and offer the potential of a more effective lower dose agent than can be administered clinically as a single bolus intravenous injection.