MAST cells and basophils bind monomeric IgE antibodies reversibly but tightly by way of specific Fc receptors on their surface. Limited bridging of bound IgE molecules by multi-valent antigen or anti-IgE antibody releases stored granules which liberate histamine and other mediators of the allergic response1. The interaction of receptor-bound IgE with antigen (or with anti-IgE antibody) and the consequent cellular response (degranulation) provides an excellent opportunity for exploring the role of the lateral mobility of defined receptors in the transmission of 'signals' across the plasma membrane. Degranulation requires a limited bridging of complexes; excessive aggregation inhibits this response2-4. We therefore measured the lateral mobility of the unperturbed IgE-receptor complexes on rat peritoneal mast cells and the effects on mobility of conditions that induce or inhibit degranulation. We redetermined5 the valence of the IgE receptor and the extent to which the IgE-receptor complexes react with each other. Fluorescence photobleaching recovery (FPR) was used to measure the macroscopic lateral motion of fluorescent cell-surface components6-9. We and others have used this or similar methods, to measure the mobility on cell surfaces of concanavalin A (con A) binding sites (which include various surface components) and nonspecifically labelled membrane proteins6,7,10-12. The present work and a concurrent study of acetylcholine receptors on developing myotubes (Axelrod et al., unpublished) are the first quantitative measurements of the lateral motion of specific cell-surface receptors.