The invention reveals a novel way to create mutants of streptokinase(SK) which become more resistant to hydrolytic inactivation by human plasmin (HP1m) and more effective in activation of human plasminogen (HP1g) than the native SK that is commercially available. The novel HP1m hydrolysisresistant SK can be created by the techniques of gene engineering to substitute the amino acid residues near the peptide bonds that are hydrolyzed by HP1m. Since Lys59-Ser60 is among the few peptide bonds which are cleaved in the early reaction with HP1m and the NH2-terminal peptide chain of Ile1-Lys59 is essential in stabilizing the structure of SK, the mutation at/or near by the peptide bond of Lys59-Ser60 should be able to prevent the hydrolysis of the peptide bond and might improve the stability of SK as a HP1g activator. For example, a mutant SK,SK-K59E, in which the amino acid Lys59 was replaced by Glu59 was prepared to test the novel idea. The SK-K59K is more stable than native authentic SK in reaction with HP1m. The activator activity of SK-K59E decreased less than 5% after interaction with HP1m for 2 hours. On the other hand, the native SK lost 50% of its activator activity under the same conditions. The samples of SK and HP1m reaction products were analyzed by sodium dodecyl sulfate gel electrophoresis (SDS-PAGE)  and NH2-terminal sequence determination. The results indicate that the NH2-terminal peptides Ilel-Lys59 of native SK was cleaved off by HP1m, while the peptide bond Glu59-Ser60 in SK-K59E was resistant to hydrolysis by HP1m. SK-K59E can induce caseinolysis with two times larger lysis zone in the casein-HP1g plate and can also cause lysis of plasma clot more efficiently than the nonmutant native SK from the commercial source. In conclusion, as shown in the above example SK mutants in which the Lys59 is replaced by other amino acids would improve their stability in reaction with HP1m and become better activators of HP1g. In the SK mutants in which the Lys or Arg at the specific sites were substituted by other amino acids, the peptide bonds at the corresponding positions would become resistant to hydrolysis by HP1m since HP1m  can specifically hydrolyze the peptide bonds at the carboxyl side of Lys and Arg. The amino acids next to either  side of the peptide bond Lys59-Ser60 could also affect the hydrolysis of the peptide bond. Therefore, the mutation of one or more amino acids of the tetrapeptide Pro58-Lys59-Ser60-Lys61 would have similar effects. The investigation therefore represents a new technique to generate novel SK mutants as an improved thrombolytic agent by making mutation at the tetrapeptide of SK and could be used as a thrombolytic agent. The mutant SK can be used to form HP1g and native SK and could be used as a thrombolytic agent. The mutant SK sequence could be coupled with other proteins to form fusion proteins and improve the fibrinolytic activity of the fusion proteins. Some truncated SKs such as SK(16-378) could activate HP1g as efficiently as native SK. The truncated SKs comprising the modification of Pro58-Lys59-Ser60-Lys61 to other amino acids would have a better thrombolytic activity than the corresponding native truncated SK.