A new sensitive test--platelet-induced thrombin generation time (PITT)--is described, in which the formation of thrombin in partially anticoagulated platelet-rich plasma (PRP) leads to aggregation immediately followed by coagulation of PRP. 0.6 ml PRP are rotated in a disk-shaped cuvette within the light beam of a photometer. In PITT, platelets stick to the cuvette wall and, mediated by a large PRP/surface/air interface at the cuvette wall, are activated and participate in thrombin formation which leads to aggregation and clotting. The times from onset of rotation until aggregation (Ta) and until coagulation (Tc) of the PRP samples are recorded. PITT was very sensitive and detected low concentrations of unfractionated heparin (0.01 IU/ml) in vitro. PITT parameters were significantly prolonged ex vivo 2 h after oral administration of acetylsalicylic acid (0.5 g) and after single subcutaneous injections of heparin (5,000 IU). Patients receiving phenprocoumon prophylaxis had markedly prolonged Ta and Tc values (longer than 20 min, n = 23). Patients with recent thrombotic episodes had markedly shorter values than healthy volunteers. PITT may become a very sensitive global test to detect mild hemorrhagic disorders, to monitor the effects of antithrombotic drugs and to detect patients with a risk of vascular occlusions.