Streptokinase is an extracellular metallo-enzyme produced by alpha-haemolytic streptococcus and is used as an effective and cheap clot-dissolving medication in some cases of myocardial infarction (heart attack) and pulmonary embolism.

It belongs to a group of medicines known as fibrinolytics, and works by cleaving plasminogen and producing plasmin.

Plasmin is produced in the blood to break down the major constituent of blood clots fibrin, therefore dissolving clots once they have fulfilled their purpose in stopping bleeding. Extra production of plasmin caused by streptokinase breaks down unwanted blood clots, for example, in the lungs (pulmonary embolism).

It is given intravenously as soon as possible after the onset of a heart attack (acute phase - myocardial infarction) to dissolve clots in the arteries of the heart wall. This reduces the amount of damage to the heart muscle. Streptokinase is a bacterial product so the body will build up an immunity to it. It is recommended that this medicine should not be used again after four days from the first administration, as it may not be as effective and can also cause an allergic reaction. For this reason, it is usually given only for a person's first heart attack.