Western immunoblot analysis of vWF propeptide processing. Equal aliquots of conditioned medium from transiently transfected COS-1 cells were analyzed by SDS-PAGE under reducing conditions. Proteins were transferred to nitrocellulose and probed with anti-vWF antibody as described in the Materials and Methods. Where cotransfection was performed, equal amounts of the two plasmid DNAs were used in the transfection. The ratio of pro-vWF to mature was quantified by scanning as described in the Materials and Methods. The amount of pro-vWF in this analysis is underrepresented due to the poorer transfer efficiency of the high molecular weight pro-vWF. (○) Pro-vWF; (•) mature vWF.