There is a growing need to understand the impact of environmental sulfhydryl group-reactive heavy metals on the immune system. Here we show that Ag+ induces mast cell degranulation, as does the aggregation of the high affinity immunoglobulin E receptor (Fc epsilon RI). Micromolar quantities of Ag+ specifically induced degranulation of mast cell model rat basophilic leukemia (RBL-2H3) cells without showing cytotoxicity. The Ag+-mediated degranulation could be observed as rapidly as 5 min after the addition of the ions. Ag+ also induced a rapid change in tyrosine phosphorylation of multiple cellular proteins including the focal adhesion kinase but not Syk kinase. The Syk-selective inhibitor piceatannol and the Src family-selective tyrosine kinase inhibitor PP1 dose-dependently inhibited Fc epsilon RI-mediated degranulation, whereas neither compound inhibited the Ag+-mediated degranulation. Furthermore, likewise Fc epsilon RI aggregation, Ag+ also induced leukotriene secretion. These results show that Ag+ activates RBL-2H3 mast cells through a tyrosine phosphorylation-linked mechanism, which is distinct from that involved in Fc epsilon RI-mediated activation.