Abstract: Derivatives of a nonimmunogenic plasminogen activator which comprises at least one polyalkylene glycol group chemically bonded with at least one coupling agent to amino acid side chains of said plasminogen activator, wherein said polyalkylene glycol has a molecular weight of about 200-20,000 and is unsubstituted or is substituted with one or more alkyl, alkoxy or alkanoyl groups or a mixture thereof.The plasminogen activator derivatives have an extended circulating life in the mammalian bloodstream and also inhibit the formation of thrombus in the same.