To optimize conditions of acousto-enzymatic thrombolysis the influence of low-frequency ultrasound on the isolated preparation of streptokinase was investigated. The ultrasound treatment with intensity 26 W/cm2 at 37° С within 5-10 minutes was not accompanied by changes of structure-functional properties of the streptokinase molecule. Increase of ultrasound-processing time (10-60 minutes) resulted in non-covalent hydrophobic aggregation of some part of the protein. In contrast to native protein ultrasound modified streptokinase is readily degraded by plasmin with formation of polypeptide fragments with molecular weights ranged from 43 up to 14 kD. The processes of aggregation and increased proteolytic degradation resulted in lower efficiency of plasmin autoactivation under the action of sounded streptokinase.