The IgE-binding proteins from flour, associated with bakers' asthma, have been reassessed by use of a modified RAST suitable for both soluble and insoluble proteins. Nitrocellulose sheet was used for preparing RAST discs, and seven different solvents were compared for their suitability in preparing discs. Dilute alkali (1% potassium hydroxide) was chosen as the best solvent for disc preparation, and its use was compared with that of water as solvent. RAST analyses of sera from 24 allergic bakers demonstrated that the albumin fraction of flour is clearly allergenic (as found in previous studies), but in addition, major IgE-binding proteins were found in the other three fractions (globulin, gliadin, and glutenin) when potassium hydroxide was the solvent (but not with water). We conclude that current RAST procedures, which favor water-soluble allergens, are inadequate because they do not satisfactorily test for water-insoluble allergens.