Previous studies with a monoclonal antibody (designated UB7) raised against isolated haustorial complexes formed by the pea powdery mildew fungus Erysiphe pisi showed that it recognized an abundant glycoprotein found in cell walls and plasma membranes of haustoria and the surface mycelium. In this paper, Western blotting and phase-partitioning in the detergent Triton X-114 have been used to show that the plasma membrane antigens recognized by UB7 comprise a 62 kDa integral glycoprotein and a set of integral membrane glycoproteins of lower molecular weight. A 59 kDa glycoprotein recognized by UB7 has been extracted from the fungus using aqueous buffer, and the evidence suggests that this represents the cell wall form of the antigen. Binding of UB7 is abolished by pre-treatment of glycoproteins with peptide-N-glycosidase, but is retained after endo-F treatment. This suggests that the epitope to which UB7 binds is the innermost N-acetylglucosamine residue of N-linked carbohydrate side chains of glycoproteins, possibly substituted with one or more sugars. When tested for cross-reactivity with other fungi, UB7 bound to some, but not all, of those examined, and recognized different sets of glycoproteins compared with those detected in E. pisi. However, UB7 did not bind to any higher plants nor to any animal glycoproteins tested. This antibody thus identifies a carbohydrate epitope restricted to glycoproteins in a subset of the fungi.