Primary hemostasis (PH), i.e., hemostatic platelet plug formation, and the subsequent coagulation were recorded and quantified from the same nonanticoagulated venous blood sample with the use of the Haemostatometer®. In addition, platelet thrombus formation induced by interaction of flowing native blood with a collagen fiber under low shear rates (450 s−1) was simultaneously analyzed by this device. The effect of monoclonal antibodies (MoAbs) directed against von Willebrand’s factor antigen (vWF: Ag), platelet glycoprotein Ib (GPIb) and the GPIIb/IIIa complex, andfibrinogenwere studied. PH was significantly inhibited by MoAbs against vWF:Ag, GPIIb/IIIa, and fibrinogen but was unaffected by antibody against GPIb. Collagen-induced thrombosis was prevented by MoAbs against vWF:Ag and GPIb, slightly inhibited by antifibrinogen, and unaffected by blockage of platelet membrane GPIIb/IIIa. The effect of a single 600-mg dose of aspirin was monitored, and abnormal PH was still detectable five days later. From the 13 hemophiliacs tested, 7 showed significantly prolonged PH. In von Willebrand’s disease, a characteristic defect of PH with significant inhibition or absence of collagen-platelet interaction was observed in all the 11 patients. PH was greatly prolonged in both of the two patients with storage pool deficiency. The technique detected improvement of platelet function, i.e., PH in all of six patients with bleeding disorders after replacement therapy or DDAVP infusion. The authors conclude that the Haemostatometer technique is a sensitive test for determining platelet dysfunction and monitoring efficacy of factor-replacement or DDAVP therapy.