Mast cell precursors invade from the peripheral blood into local tissues where they differentiate to their mature phenotypes. However, the mechanism of this migration process has been unclear. We clearly demonstrated here the production and release of matrix metalloproteinase-9 (MMP-9), a matrix-degrading enzyme necessary for leukocyte transmigration, by interleukin-3–dependent mouse mast cell progenitors: bone marrow-derived cultured mast cells and IC-2 mast cells. Because several interleukin-3–independent mast cell lines with active mutations in the c-kit gene did not release MMP-9, the possible involvement of c-kit receptor activation in downregulation of MMP-9 production was predicted. c-kitreceptor activation by stem cell factor led to a significant decrease in MMP-9 production of cultured mast cells and IC-2 mast cells transfected with the c-kit gene. Thus, the present results suggest that mast cell precursors are able to produce MMP-9, which may be essential for mast cell migration into tissues, and that stem cell factor may downregulate the MMP-9 production, resulting in engagement of mast cells to matrix components.