Positive RAST (> 5% radioactive uptakes) to wheat endosperm proteins were found in approximately one-quarter of subjects who had both a positive skin prick and RAST (> 10% radioactive uptake) to ryegrass pollen proteins. Immunoblotting of proteins electrophoretically transferred to nitrocellulose membrane after SDS-PAGE of ryegrass pollen and wheat endosperm proteins confirmed the crossreactive properties of the sera identified by RAST testing. Immunoadsorption of serum IgE onto nitrocellulose membrane, to which ryegrass pollen or wheat endosperm proteins had been adsorbed, removed IgE from crossreactive sera reactive to both ryegrass pollen and wheat endosperm proteins. Elution of the adsorbed IgE from the nitrocellulose membrane after immunoadsorption and probing blotted strips of both ryegrass pollen and wheat endosperm proteins supported the results obtained from the immunoadsorption experiments. This data provides evidence that the crossreactivity of IgE antibodies in sera reacting with both ryegrass pollen and wheat endosperm proteins involves common or related determinants and has implications for the clinical management of these allergic subjects.