SummaryPresent knowledge oi; the chemistry, distribution and inhibition of heparin is summarized; in spite of the high concentration of hepan in human tissues, its physiological function is still unclear. The recommendation to use heparin in thromboembolic conditions is based on its anticoagulant effect. Heparin does however stimulate platelet aggregation, possibly related to a liberation of thromboxaneA.:,thrombocytopenia has occasionally been observed following its use in man. The main clinical indications for the clinical use of heparin are briefly discussed.