This review suggests a model in which both beta- and gamma-chains synergize in the initiation of Fc epsilon RI signal transduction function. Receptor aggregation by antigens induces activation of lyn, which is already bound to the Fc epsilon RI beta-chain under resting conditions. Whilst activated, lyn would phosphorylate the tyrosine residues in the Fc epsilon RI gamma-chain. This phosphorylation would be responsible for the recruitment of syk (probably via its SH2 domains) as well as other signalling molecules. Syk kinase would then be activated by the engagement of its SH2 domains and/or its phosphorylation. Syk could then interact with and activate (through phosphorylation) downstream effector molecules.