Thirty-four patients with acute myocardial infarction were treated prospectively using a new strategy of prehospital intravenous streptokinase given by a physician-operated mobile intensive care unit. The 29 prehospital-treated patients who had experienced no previous myocardial infarction were compared to a similar group treated with streptokinase inhospital. Patients receiving streptokinase in the prehospital phase of acute myocardial infarction had smaller infarcts and better residual myocardial function than the group given streptokinase inhospital in terms of peak creatinine phosphokinase, ejection fraction, computer-derived dysfunction index, and electrocardiographic QRS score. The only difference between these groups at baseline was the duration of pain prior to initiation of streptokinase therapy. There were no major complications related to prehospital administration of streptokinase.