Factor VIII, a high molecular weight glycoprotein complex which has an important role in haemostasis, consists of two immunologically as well as functionally discernible moieties that can be isolated separately. These are factor VIII/von Willebrand factor (FVIII/vWF) which is associated with the factor VIII-related antigen (FVIIIRAg), and the factor VIII/procoagulant activity (FVIII/C) which is associated with the factor VIII/procoagulant antigen (FVIII/CAg). The FVIII/C activity is decreased or absent in patients with haemophilia A (for a review of the structure and function of the factor VIII complex, see refs 1 and 2). Immunological techniques, combined with cell culture, have demonstrated that FVIIIRAg is present in and synthesized by endothelial cells and megakaryocytes. However, the organ and/or cell type responsible for the production of FVIII/C has not been established. Indirect evidence derived from organ transplantation in experimental animals suggests that the liver is the most likely organ for FVIII/C production. Here we have used a monoclonal antibody against FVIII/CAg in combination with a sensitive immunostaining technique to demonstrate the presence of FVIII/CAg in hepatic sinusoidal endothelial cells.