Immunogold staining was used to study the ultrastructural distribution of von Willebrand factor (vWF) in unstimulated platelets. vWF was detected in the alpha-granules with a specific eccentric distribution pattern opposite the nucleoids. Similar findings were obtained with a polyclonal antibody or a pool of monoclonal antibodies to human vWF. This labeling coincided with the presence of tubular structures located at the periphery of the alpha-granules. These structures were better visualized on platelets treated for standard electron microscopy: they formed a group of one to four tubules ranging from 200 A to 250 A in diameter. They closely resembled the internal tubular structures found in Weibel-Palade bodies, which are the storage organelles of vWF in endothelial cells.