Immunoglobulin E comprises the main immunoglobulin class associated with allergy. Its multifarious activities are mediated by two types of Fc receptors found on different cell populations, Fc epsilon R1 on mast cells and basophils, and Fc epsilon R2 on inflammatory cells (monocytes, eosinophils and platelets) and B lymphocytes. Recombinant epsilon-chain fragments synthesized in Escherichia coli have provided the means of mapping the receptor-binding sites on human IgE, and blocking IgE-receptor interactions. We have previously shown that the Fc epsilon R1 binding site is contained within a sequence (Gln 301-Arg 376) spanning the C epsilon 2 and C epsilon 3 domains. Here we show that Fc epsilon R2 can recognize a motif in the C epsilon 3 domain that is formed on dimerization of one or both of the flanking (C epsilon 2 and C epsilon 4) domains. Glycosylation of IgE is not required for the activity of either receptor.