Blumeria graminis f.sp. tritici (syn. Erysiphe gruminis f.sp. tritici) causes an important disease of wheat (powdery mildew) to which Hordeum uulgare and H. chilense are resistant. The study of chromosomal addition lines of H. uulgare and H. chilense in wheat showed that they possessed resistance to wheat powdery mildew. This was expressed as a reduction of disease severity but it was not associated with increased macroscopically visible necrosis. The resistance is of broad genetic basis, conferred by gene(s) present on different chromosomes of both H. vulgare and H. chilense. The feasibility of transferring this resistance to wheat is discussed.