Hirudin, a 65 amino acid polypeptide form the medicinal leech, is an extremely efficient and specific thrombin inhibitor whose therapeutic potential has been demonstrated in a number of animal models. We have developed protocols for the production of recombinant hirudin by secretion from S. cerevisiae and carried out a full biologic evaluation of the purified product. These studies showed that natural and recombinant hirudin was similar in structure and in biologic function in vitro. Moreover, the recombinant protein displayed strong antithrombotic activity in several experimental thrombosis models in vivo, confirming the molecule's promise in the therapy of thrombotic disorders.