Antithrombotic, antiarteriosclerotic, and anti-inflammatory actions of heparins may be mediated by binding of heparin to leukocytes. To get the first information on this hypothesis, fluorescein-labeled LMMH-tyramine has been used (LMMH-tyramine-FITC) to analyze the binding of GAGs to lymphocytes, monocytes, or granulocytes. The fluorescence intensity on leukocytes was quantified by flow cytometry analysis. LMMH-tyramine-FITC bound dose dependently to lymphocytes, monocytes, and granulocytes. PE-labeled CD 11c antibodies identified the specificity of the binding of LMMH-tyramine-FITC to granulocytes. UFH and LMMH displaced LMMH-tyramine-FITC dose dependently from leukocytes. Equimolar ratios of LMMH and LMMH-tyramine-FITC revealed a 50% displacement of the labeled compound, indicating the specific binding by the polysaccharide chain. The synthetic pentasaccharide was 10-fold and dermatan sulfate 100-fold less effective than UFH in displacing LMMH-tyramine-FITC from leukocytes. The data indicate that negatively charged GAGs bind to the surface of lymphocytes, monocytes, and granulocytes. By decreasing the number of negatively charged groups of GAGs, the binding to the surface of leukocytes is decreased. The data obtained with the synthetic pentasaccharide indicate a binding of GAGs to the surface of leukocytes, independently of AT III. The cellular binding of heparins may significantly contribute to its antithrombotic and other biologic activities.