A modified capillary thrombometer was constructed to study the rate of thrombus formation using heparinized whole blood (2 U/ml) from 4 different groups of pigs: normal, heterozygous von Willebrand's disease (vWD), homozygous vWD, and platelet storage pool disease (SPD). The median thrombosis times for the 4 groups of pigs were: 5.3 min (range = 3.0-14), 31 min (range = 4.0-47), 55 min (range = 41-60), and 60 min (range = 15-60), respectively. Significant differences were demonstrated between all pig groups (p less than .01 - p less than .001), except between the homozygous vWD pigs and the SPD pigs (p = 0.8), both of which are clinical bleeders. Cryoprecipitate was infused into 3 pigs with homozygous vWD. Partial correction of the capillary thrombometer thrombosis time and the in vivo ear bleeding time was observed. Murine monoclonal antibodies to porcine von Willebrand factor were added to normal pig whole blood samples in the capillary thrombometer. Four of six antibodies prolonged the thrombosis time and had similar effects on the ear bleeding time. Using these monoclonal antibodies, an immunoperoxidase stain demonstrated plasmatic and platelet associated von Willebrand factor in sections of thrombi from the capillary thrombometer. These experiments confirm that von Willebrand factor is important to thrombus formation in the capillary thrombometer and that measurements by this instrument may relate to in vivo hemostasis as measured by the ear bleeding time.