Genetics studies have identified the gene for the high-affinity IgE receptor (FC(epsilon)RI) beta subunit as a candidate gene for atopy. We have shown that beta is an intrinsic signaling amplifier leading to enhanced allergic responses in vivo. Here we report that beta has a second amplification function: the amplification of Fc(epsilon)RI cell surface expression. This function is due to an early association of beta with alpha, resulting in improved trafficking and maturation of alpha and receptor complexes. These data provide a possible molecular explanation for the large difference in Fc(epsilon)RI density between beta-cells such as monocytes, dendritic cells, and beta+ effector cells (mast cells, basophils). In beta+ cells, the combined signaling and expression amplification results in an estimated 12- to 30-fold amplification of downstream events.