A method is provided for preparing a biologically active molecule having an increased serum half-life. The method involves conjugating a polymer such as polyethylene glycol to the biologically active molecule. Also provided are polypeptide drugs having an increased serum half-life, e.g., human urokinase plasminogen activator (human "uPA" or "hUPA") or a fragment or derivative thereof. Pharmaceutical compositions containing such molecules and methods of using them to treat uPA-mediated and uPA receptor-mediated disorders are also provided.