Factor VIII-related antigen was localized ultrastructurally in a variety of human tissues (smooth muscle, skeletal muscle, breast, capillary hemangioma) with the use of a low-temperature embedding protein A-gold technique with both polyclonal and monoclonal antisera directed against von Willebrand factor. All endothelial cells examined localized the anti-von Willebrand factor to Weibel-Palade bodies. Cisternae of the endoplasmic reticulum, and cytoplasmic vacuoles were also labeled. These results establish the distribution of factor VIII-related antigen at the subcellular level. The observed distribution suggests that the endothelial cells synthesize von Willebrand factor, store it in Weibel-Palade bodies, and release it by exocytosis. These observations provide in vivo confirmation for previous biochemical and immunocytochemical data obtained from studies on cultured endothelial cells.