Three experimental models have been employed to investigate the mechanism of the prolonged bleeding time in patients with von Willebrand's disease (vWd). 1-deamino-8-D-arginine vasopressin (DDAVP), a synthetic analogue of the antidiuretic hormone, was administered to normal volunteers and patients with vWd in order to induce a short-term, endogenous increase of factor VIII procoagulant activity (VIIIAHF), factor VIII-related antigen (VIIIAGN), and Willebrand factor (VIIIVWF); and to investigate the relationship between bleeding time and plasma variations of factor VIII-associated properties. In normal subjects DDAVP administration was followed by a marked increase of VIIIAHF, VIIIAGN, and VIIIVWF; yet the bleeding time remained unchanged. The same parameters were also raised in two groups of patients with vWd. In a third group of patients with severe recessive vWd, factor VIII-associated properties, which were not measurable before the infusion, were unmodified. The bleeding time remained unchanged in all vWd patients. To investigate the effect of the exogenous increase of factor VIII-associated properties, cryoprecipitate was given to ten vWd patients before dental surgery. Despite the marked increase of VIIAHF, VIIAGN, and VIIVWF observed after the infusion, bleeding time was not shortened. Finally, in order to evaluate the hypothesis that factor VIII may exert its effect on primary hemostasis locally in the vessel wall, VIIIAGN and its relationship with the bleeding time were studied by direct immunofluorescence in gum-biopsy specimens obtained in vWd patients before cryoprecipitate infusion. No reaction could be elicited in five patients with severe, recessive vWd, whereas venules and arterioles stained positively in five patients with a moderate form of the disease. Immunofluorescence microscopy was also carried out in specimens obtained after cryoprecipitate at a time when the plasma defects were corrected but the long bleeding time was not modified; no reaction was detectable on the vessel wall of the three patients who were negative before the infusion.