The gene for a human epsilon chain Fc fragment has been cloned and expressed at a high level in Escherichia coli, and its biological activity in binding to the high-affinity receptors on mast cells and basophils and mediating histamine release has been examined in a variety of assays, including the inhibition of passive cutaneous anaphylaxis in human skin, which was induced by ragweed immunoglobulin IgE antibody and antigen. The positive results obtained in these assays encouraged us to try to analyse the binding site on IgE by site-directed mutagenesis. We describe deletion mutants here that narrow down the binding site on IgE for the mast cell receptor to a stretch of 76 amino acids (residues 301-376 on the ND epsilon chain) spanning the CH2 and CH3 domains. This peptide displays activity in the human skin test indistinguishable from that of a myeloma IgE.