Diagrammatic representation of isolates of FMD virus with neutralising epitopes designated by letters A to M. The reference FMD virus strains 1, 2 and 3 have no neutralising epitopes in common. The field FMD virus strains X, Y and Z each share one neutralising epitope with reference strain 1 but have no neutralising epitopes in common. Field strain X has neutralising epitopes in common with all three reference strains.