Suggestive evidence indicates that immunoglobulin E (IgE)-dependent activation of mononuclear phagocytes plays an important pathogenic role in allergic tissue inflammation. Prevailing opinion holds that low affinity IgE receptors are the relevant IgE-binding structures on monocytes/macrophages and that functional events occurring after cross-linking of membrane-bound IgE on these cells are mediated by these receptors. Here we demonstrate that peripheral blood monocytes can bind monomeric IgE via the high affinity IgE receptor (Fc epsilon RI) and that Fc epsilon RI expression on these cells is upregulated in atopic persons. Further, we demonstrate that, upon monocyte adherence to substrate, bridging of monocyte Fc epsilon RI is followed by cell activation. We propose that direct interaction of multivalent allergen with Fc epsilon RI(+)-bound IgE on mononuclear phagocytes results in cell signaling via Fc epsilon RI and that the biological consequences of this event may critically influence the outcome of allergic reactions.