The lumenal plasma membrane of most mammalian urothelia possesses discrete polygonal plaque regions which are characterized by particles projecting through the lumenal leaflet and bridging the membrane interior. Fractions enriched in plaque regions were prepared from sheep urinary bladders and were used as antigen. A 50% saturated ammonium sulphate fraction of immunized rabbit serum showed at least two antibodies to plaque region components in Ouchterlony diffusion test. Intact sheep urothelium was treated with the antibody preparation and labelling was visualized by treatment with peroxidase-conjugated goat anti-rabbit antibody followed by peroxidase reaction. Control tissue treated with non-immune globulin showed no reaction product. The plaque-specific antibody preparation was also shown to stain the lumenal leaflet of isolated sheep membranes. Further, cross reactivity of sheep plaque antibody was demonstrated with rat and rabbit membranes. The possibility that antibodies to specific components in sheep urothelial membranes may be used diagnostically to detect changes in urothelial plasma membranes of other species is discussed.