Streptokinase (SK) is an enzyme secreted by several species of streptococci that can bind and activate humanplasminogen. SK is used as an effective and inexpensive thrombolysis medication in some cases of myocardial infarction(heart attack) and pulmonary embolism. Streptokinase belongs to a group of medications known as fibrinolytics, and complexes of streptokinase with human plasminogen can hydrolytically activate other unbound plasminogen by activating through bond cleavage to produce plasmin. There are three domains to Streptokinase, denoted α (residues 1–150), β (residues 151–287), and γ (residues 288–414). Each domain binds plasminogen, although none can activate plasminogen independently.

It is on the World Health Organization’s List of Essential Medicines, a list of the most important medication needed in a basic health system.