Abstract: CD23 is a low affinity receptor for IgE. The extracellular part of this molecule has been linked to a modified leucine zipper. This results in a trimeric configuration, since the stalk region of the CD23 molecule is itself a weak leucine zipper. This chimeric protein, designated LZ-CD23, interacts with IgE in a much stronger fashion than either simply the extracellular domain alone or the intact membrane CD23. This is the result of an increased avidity due to the stable trimeric configuration. The chimera has approximately at 10,000 fold increased ability to block IgE binding to mast cell/basophils, compared to native soluble CD23. At approximately equal concentrations to added IgE, 80% inhibition of IgE binding to Fc.di-elect cons.RI on mast cells was obtained. Thus, the LZ-CD23 chimera provides a new means for blocking and binding which provides benefits in the treatment of IgE-mediated asthma and allergic diseases.