The investigations on LMW heparins that are reviewed in this article demonstrate that there is no relationship between the specific anti-Factor Xa activity and the release of HTGL activity after intravenous or subcutaneous administration to man. The measurement of anti-Factor Xa-like activity in patients treated with unfractionated or with LMW heparin was validated in whole blood samples obtained from the fingertip using Heptest reagents. Administration of high doses of LMW heparin twice daily resulted in a small but significant accumulation in healthy persons. The half-life of anti-Factor Xa-like activity increased after repeated administration, indicating that glycosaminoglycans or other compounds are released by LMW heparin into the bloodstream. The studies indicated that the improved pharmacologic properties may induce fewer side effects than unfractionated heparin and that patients with contraindications to unfractionated heparin or oral anticoagulants may be given long-term treatment with LMW heparin.