The antigenic structure of the coat protein (CP) of potato mop-top furovirus (PMTV) was studied by electron microscopy of virus particles labeled with gold-conjugated monoclonal antibodies (MAbs) and by the reactions of MAbs with overlapping octapeptides (Pepscan) representing the complete amino acid sequence of the CP. A total of seven epitopes were identified in the CP. MAb SCR 69 detected a continuous epitope, which was located at the extreme N-terminus of the CP, was exposed at the surface along the sides of PMTV particles, and was removed by treating them with trypsin. MAb SCR 68 detected a discontinuous epitope found at the concave end of PMTV particles. Five other epitopes, which were detected by Pepscan tests, were located internally in, and at intervals along, the CP amino acid sequence. A tentative model of the PMTV CP subunit was produced, based on computer-aided prediction of its secondary structure and apparent similarities with the CP of tobacco mosaic virus. In this model, four of the epitopes occur at high radius in each of the pairs of parallel and anti-parallel alpha-helices in the CP subunit. The fifth is at low radius in the putative left radial alpha-helix. The epitope detected by MAb SCR 77, although amenable to study by Pepscan, contains three reactive elements, separated by short runs of nonessential residues, in a sequence of 13 amino acids. In intact virus particles, the CPs of beet necrotic yellow vein furovirus and PMTV apparently differ in the accessibility of their N- and C-termini.