A 20,000 molecular weight (MW) homologous restriction factor (HRF20), detected by 1F5 monoclonal antibody (mAb), is present on blood cell surfaces and inhibits the terminal stage of the formation of membrane attack complexes by homologous complement activation. The tissue distribution of HRF20 was studied by immunohistochemical analysis using 1F5. HRF20 was predominantly expressed on endothelial cells of systemic arteries, veins and capillaries, as well as on the surface of cultured human umbilical vein endothelial cells. HRF20 was also detected, to a lesser extent, on the Schwann sheath of peripheral nerve fibres, ependymal cells and certain epithelial cells such as acinar cells of the salivary gland, bronchial epithelium, renal tubules and squamous epithelium. The distribution pattern of HRF20 differed somewhat from that of decay-accelerating factor (DAF), which is another membrane inhibitor of homologous complement activation.