Background: Soy lecithin is widely used as an emulsifier in processed foods, pharmaceuticals and cosmetics. Soy lecithin is composed principally of phospholipids; however, it has also been shown to contain IgE-binding proteins, albeit at a low level. A few clinical cases involving allergic reactions to soy lecithin have been reported. The purpose of this investigation is to better characterize the IgE-binding proteins typically found in lecithin. Methods: Soy lecithin proteins were isolated following solvent extraction of lipid components and then separated on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The separated lecithin proteins were immunoblotted with sera from soy-sensitive individuals to determine the pattern of IgE-binding proteins. The identity of IgE-reactive bands was determined from their N-terminal sequence. Results: The level of protein in six lecithin samples obtained from commercial suppliers ranged from 100 to 1,400 ppm. Lecithin samples showed similar protein patterns when examined by SDS-PAGE. Immunoblotting with sera from soy-sensitive individuals showed IgE binding to bands corresponding to 7, 12, 20, 39 and 57 kD. N-terminal analysis of these IgE-binding bands resulted in sequences for 3 components. The 12-kD band was identified as a methionine-rich protein (MRP) and a member of the 2S albumin class of soy proteins. The 20-kD band was found to be soybean Kunitz trypsin inhibitor. The 39-kD band was matched to a soy protein with unknown function. Conclusions: Soy lecithin contains a number of IgE-binding proteins; thus, it might represent a source of hidden allergens. These allergens are a more significant concern for soy-allergic individuals consuming lecithin products as a health supplement. In addition, the MRP and the 39-kD protein identified in this study represent newly identified IgE-binding proteins.