Monoclonal antibodies to A, B and H blood group isoantigens (BGI) were used in an indirect immunoperoxidase technique to compare the distribution of BGI expression as a marker of vascular endothelial cells with that of coagulation factor VIII related antigen (F VIII-RAg), a well established marker for vascular endothelium. Formalin-fixed and paraffin-embedded material from 70 specimens representing a wide range of reactive and benign or malignant neoplastic states of the vascular endothelium was used to provide adjacent serial tissue sections to compare directly the tissue expression of BGI and of F VIII-RAg. In reactive and benign neoplastic conditions of the vascular endothelium, cell membrane expression of the BGI appropriate to the patient's blood group was readily detected. In the more cellular and proliferative areas of some haemangiomas, pyogenic granulomas and cases of angiolymphoid hyperplasia with eosinophilia, BGI proved a superior marker of the endothelial cells compared with F VIII-RAg. The malignant endothelial cells of 27 specimens of angiosarcoma showed deletion of BGI expression in 24 and of F VIII-RAg in 19. Expression of neither antigen showed any significant correlation with the degree of tumour differentiation. Some cases of lymphangioma, uniformly negative for F VIII-RAg staining, showed strong expression of BGI. The possible use of BGI expression as a marker of vascular endothelium is discussed in the light of the known tissue expression of F VIII-RAg.