The systemic effects and complications associated with the infusion of low-dose (5,000 U/hr) intravascular streptokinase were reviewed in 159 patients. Eighty-two percent of the patients had a 50% decrease in plasma fibrinogen levels during the first 4 hours of treatment, and 100% of the patients showed the same decline following 12 hours of treatment. The thrombin time was prolonged to at least 1 1/2 times the control thrombin time in 42% of the patients at 4 hours of treatment and in 92% at 24 hours of treatment. The rate or degree of change in fibrinogen levels or prolongation of the thrombin time did not correlate with the incidence of complications. Seventeen patients (10.7%) suffered major complications. Thirteen of the 17 had hemorrhagic complications, three had thrombotic or embolic complications, and one had a false aneurysm. In this series, systemic effects occurred in all patients. It is concluded that, despite precautionary measures, hemorrhagic complications may occur. Steps should be taken to minimize complications, including careful observation of the patient and monitoring of plasma fibrinogen levels until adequate regeneration is complete.