Radioallergosorbent testing (RAST) of sera from subjects sensitized to wheat and rye flour indicated that there is significant reaction with seed extracts of 12 cereals (wheat, durum wheat, triticale, cereal rye, barley, rye grass, oats, canary grass, rice, maize, sorghum and Johnson grass). Results were evaluated in terms of taxonomic relationships and of the electrophoretically determined protein composition of the cereal extracts. RAST uptakes were uniformly low in sera from four rhinitic bakers, yet were significantly above the levels for non-allergic and cord sera. Much higher RAST uptakes were obtained with sera from four asthmatic bakers when tested with wheat and its close relatives, but there was still reasonably high reactivity with more distantly related cereals. RAST inhibition experiments indicated in a more direct way the extent of cross-reactivity between grain extracts of wheat, rye, barley and oats. One baker had a history of more severe attacks of breathlessness following inhalation of rye flour compared with wheat flour. This was confirmed by bronchial challenge testing, but the comparison was not obviously consistent with the results of prick testing or estimation of histamine released from his leucocytes. The results as a whole suggested that the bran layers of cereal grains are at least as allergenic as flour.