Streptokinase is used worldwide as thrombolytic agent. Indications are deep vein thrombosis, arterial thrombosis and embolism, arteriovenous cannula occlusions, acute coronary artery thrombosis, and acute renal vein thrombosis. The major drawback in comparison with other thrombolytic drugs is its antigenicity. Allergic reactions have been reported but the underlying mechanisms have not yet been fully elucidated. A 58 year-old man with acute myocardial infarction developed an anaphylactic reaction immediately after commencement of streptokinase infusion, although immunological laboratory findings were not affected. The patient had no history of prior exposure to streptokinase, infections with streptococci, or chronic allergic reactions. 2 months after streptokinase therapy increased plasma levels were detected of specific IgG antibodies to streptokinase. The levels of specific IgE antibodies to streptokinase and other anti-streptococcal antibodies were within the normal range. A prick test performed 4 months later with 100 I.U. streptokinase was unremarkable. The problems of such allergic reactions are discussed on the basis of this case report and a review of the literature.