A series of 45 lymph nodes involved by Hodgkin's disease have been examined by means of the unlabelled antibody peroxidase-antiperoxidase (PAP) method for the demonstration of factor VIII-related antigen (F VIII R Ag). In addition, five lymph nodes showing reactive follicular hyperplasia were studied. In the specimens of nodular sclerosing Hodgkin's disease, many blood vessels were present, as demonstrated by virtue of F VIII R Ag in endothelial cells. These vessels were present within and, especially, around the nodules in this subtype; in addition the fibrous trabeculae were densely vascular. High vascularity was also a feature of specimens of the cellular variant of nodular sclerosing Hodgkin's disease. However, the other three Rye subtypes of the disease showed a striking paucity of F VIII R Ag-positive blood vessels. The reactive nodes showed numerous vessels around and between follicles but the lymph sinuses were consistently negative. Mast cells were present in all specimens but were especially frequent within and adjacent to the sinuses in reactive specimens; they were strongly positive for the F VIII R Ag, their staining being abolished by pre-adsorption of the primary antiserum with factor VIII itself.