Barley yellow dwarf (BYDV) is a group a closely related viruses which cause economic losses in a wide range of graminaceous species throughout the world. Barley plants can be protected from the effects of BYDV by the ‘Yd2’ resistance gene. Plants which contain the ‘Yd2’ gene also contain a constitutively expressed polypeptide which was not found in any plants without ‘Yd2’. Conversely, BYDV susceptible plants contain another constitutively expressed polypeptide which was not found in any of the BYDV-resistant lines examined. These two polypeptides appear to have the same molecular weight (as assessed by SDS-PAGE) and only slightly different iso-electric points. They also appear to contain an extensive range of similar antigenic determinants. Both polypeptides were found in F1 hybrids made from resistant and susceptible plants. We suggest that these two polypeptides are the products of two allelic genes. Analysis of near-isogenic lines showed that the locus which controls the ‘Yd2’ resistance gene and the locus controlling the synthesis of the two polypeptides may be within ± 9 cM of each other. We have developed a Western blot technique which allows assessment of barley lines, 4-days after seed imbibition, for the presence of the ‘Yd2’ gene.