Since mast cells play pivotal roles in allergic inflammations, we investigated how IgE-mediated stimulation modulated mast cell matrix metalloproteinase (MMP)-9 production, and its enzymatic activation. In this study, we clearly demonstrated that proMMP-9 released from murine bone marrow-derived cultured mast cells (BMCMC) was activated to its valid form after crosslinking of surface immunoglobulin (Ig)E. Serine protease inhibitors sensitive to chymases inhibited the phenomenon, indicating that certain chymases may be responsible for activation of proMMP-9. Although binding of IgE to its specific receptors did not alter MMP-9 production, the IgE crosslinkage increased both expression of mRNA, and production of MMP-9 in mast cells. Glucocorticoid declined extra cellular processing of proMMP-9 without affecting mRNA expression. These findings give rise to the possibility that production and activation of mast cell MMP-9 may be increased in the affected sites, thereby resulting in an exacerbation of tissue degradation in inflammatory conditions.