Twelve normal subjects were injected with 5000 U of commercial mucous heparin with or without preloading of 1.0 g aspirin, and 0.2 mg/kg MD 805, an arginine derivative, which is a new synthetic compound with an extremely strong affinity for thrombin, at an interval of 4 weeks after each injection. Heparin injection with or without aspirin significantly increased platelet factor 4 release. In contrast, the preloading of aspirin significantly inhibited the decrease of platelet count and the elevation of /8 thromboglobulin induced by heparin. However, MD 805 had no effect on platelet release proteins, and adequate anticoagulation by APTT was still present 60 min after the injection. MD 805 shows no stimulative effects on platelets such as with heparin.In the case of the patient's study, three patients complicated with heparin induced thrombocytopenia plus thrombus formation in the extracorporeal circulation during hemodialysis, and were treated with MD 805 instead of heparin. The platelet counts in those patients quickly returned to within the normal range, and adequate anticoagulation was obtained in the following hemodialysis sessions and no further bleeding or clot formation was noted.In conclusion, MD 805 may represent a useful alternative anticoagulant in patients with heparin induced thrombocytopenia.