Immunoelectronmicroscopy of human platelet alpha-granules reveals that von Willebrand factor (vWf:Ag) colocalizes with a small number of discrete tubular structures which appear identical to those observed within the Weibel-Palade bodies of endothelial cells. Although it is likely that tubules are composed of vWf:Ag as they are absent in severe vWD porcine platelets, their exact structural and functional nature is still unclear. In this study quantitative/qualitative analysis of vWf:Ag was undertaken in a series of platelet preparations obtained from normal pigs, normal humans and various vWD patients. Electron microscopy confirmed that normal pig platelet alpha-granules contain numerous, regularly spaced tubular structures eccentrically located and coincident with immunogold staining of vWf:Ag. In contrast, normal human platelet alpha-granules contain significantly fewer tubules (usually four to six) which are absent or reduced in number within various vWD platelet sections. Furthermore, the pig platelet lysates not only contained a full complement of multimers but also demonstrated significant intense staining of ultra-high MW material, irrespective of the presence or absence of proteolytic inhibitors. This ultra-high MW vWf appears similar to that observed within lysates prepared from endothelial cells and is susceptible to degradation to lower MW multimers. This study suggests that the tubular structures within alpha-granules and Weibel-Palade bodies may be composed of, or structurally related to, the ultra-high MW intracellular form of vWf:Ag.