We have conducted radio allergosorbent test (RAST), competitive RAST inhibition, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting using sera from patients with wheat-induced asthma to investigate the immunological relationship between wheat, rye, barley and soya, and to identify common proteins between these flours. RAST showed strong associations between the levels of specific IgE to wheat flour and those of rye and barley flour. Competitive RAST inhibition showed that wheat, rye, barley and soya flours contained crossreacting proteins, in decreasing concentrations. Wheat, rye and barley flours had similar protein profiles on gel electrophoresis. Soya flour contained a number of high molecular weight proteins not present in the other cereals. Western blotting using sera from 21 wheat flour hypersensitive individuals identified a large number of allergens in the different flours. Proteins of 69, 33, 26, 21 and 12 kDa were identified as major wheat flour allergens. Rye flour proteins of 21 and 12 kDa, and barley flour proteins of 69, 52 and 10 kDa were the major allergens identified by serum from wheat hypersensitive individuals. The major common protein of soya and wheat flour had a molecular weight of 21 kDa. The majority of crossreacting allergens identified between the different flours have molecular weights similar to those of known flour enzymes or enzyme inhibitors.