It is known that heparin which is strongly anionic will precipitate in vitro with cationic proteins such as melittin, the principal constituent of bee venom. Our experiments indicate that mice immunized with complexes of melittin and heparin produced greater amounts of IgE and IgG antibody (to this protein) than did mice immunized with melittin alone. It is possible that heparin released from mast cells following a bee sting could complex with melittin and enhance the allergenic properties of this molecule.