The specific interaction of concanavalin A, the phytohemagglutinin of the jack bean (Canavalia ensiformis), with several glycoproteins has been studied. It is demonstrated that concanavalin A forms precipitates with purified monoclonal IgM and the glycoprotein phytohemagglutinins (lectins) from waxbean and soybean. It is suggested that the carbohydrate residues reactive with concanavalin A are situated on the surface of these glycoproteins.