The low-affinity receptor for IgE (FcERII or CD23) is a membrane 45-kD glycoprotein which is cleaved by an autoproteolytic mechanism into soluble 37-, 33- and 25-kD fragments that are capable of binding to IgE (IgE-binding factors, IgE-BFs). FcERIIa (which is expressed only on fresh B cells) differs from FcERIIb (which is expressed on IL4-stimulated B cells, monocytes, eosinophils and T cells) by a few intracytoplasmic amino acids. The only function of FcERII which is clearly demonstrated is the IgE-dependent cytotoxicity of FcERIIb on monocytes and eosinophils. We here review our recent observations indicating that 37-kD IgE-BFs regulate the synthesis of human IgE. Recombinant 37-kD IgE-BFs increase the IL4-induced synthesis of IgE by peripheral blood lymphocytes, as well as the IL4-independent, ongoing synthesis of IgE by either in vivo activated B cells from allergic patients or by in vitro IL4-preactivated B cells.