Castor beans contain one of the most potent allergens known to man. Previous studies suggested that these allergens are heat-stable; but our current data suggest that both heat-stable and labile allergens are present in castor bean extracts. The heat-labile allergen is of large molecular weight and is immunochemically distinct from the smaller molecular weight heat-stable allergen. Both allergens have different isoelectric points. These antigens may be clinically relevant since coffee workers sensitized to castor have IgE antibodies to both allergens.