The purpose of this study was to establish whether complement activation is the cause of transient hypotension during streptokinase infusion in patients with acute myocardial infarction. Thirteen patients with suspected acute myocardial infarction and treated with streptokinase were included. Complement 3d (C3d) as an indicator of complement activation was assessed in venous blood and blood pressure was measured. Ten patients had verified myocardial infarction. Three patients did not. Five patients developed an increase in C3d blood levels of more than 200% within 30 min. All patients developed a transient fall in blood pressure during infusion of streptokinase. This was related neither to presence of myocardial infarction nor to degree of complement activation. The mechanism of hypotension observed following the administration of streptokinase is not through complement activation, and thus how streptokinase induces hypotension is still obscure.