FcεRI activation of mast cells is thought to involve Lyn and Syk kinases proximal to the receptor and the signaling complex organized by the linker for activation of T cells (LAT). We report here that FcεRI also uses a Fyn kinase–dependent pathway that does not require Lyn kinase or the adapter LAT for its initiation, but is necessary for mast cell degranulation. Lyn-deficiency enhanced Fyn-dependent signals and degranulation, but inhibited the calcium response. Fyn-deficiency impaired degranulation, whereas Lyn-mediated signaling and calcium was normal. Thus, FcεRI-dependent mast cell degranulation involves cross-talk between Fyn and Lyn kinases.