Abstract
Insoluble conanavalin A and wheat germ agglutinin (WGA) were found to bind to carbohydrates on radio-labelled carcinoembryonic antigen (CEA). Binding by WGA was inhibited both by N-acetyl D-glucosamine and fragments of antibody to CEA, but was increased by intact antibody to CEA. This suggests that WGA binds to exposed N-acetyl D-glucosamine determinants on 125I CEA and also on antibody molecules. It also suggests that 125I CEA contains binding sites for anti-CEA which contain N-acetyl D-glucosamine as well as others which do not. Molecules of 125I CEA which bound to the insoluble lectins were more antigenic for anti-CEA than unbound molecules. These results suggest that the principal antigenic site on CEA contains N-acetyl D-glucosamine and may help to explain the agglutination of tumour cells by lectins.