Accumulating evidence strongly supports the view that the plasma membrane participates in transmembrane signaling by IgE-receptors (IgE-Fc epsilon RI) through the formation of lipid-based domains, also known as rafts. Ongoing biochemical and biophysical experiments investigate the composition, structure, and dynamics of the corresponding membrane components and how these are related to functional coupling between Fc epsilon RI and Lyn tyrosine kinase to initiate signaling in mast cells. (C) 1999 Elsevier Science B.V. All rights reserved.