When mast cells are activated they can respond by releasing their secretory granule compounds, including mast cell-specific proteases of chymase, tryptase and carboxypeptidase A (MC-CPA) type. MC-CPA is a dominant protein component of the mast cell granule and the MC-CPA gene is extremely highly expressed. Despite this, relatively little has been known of its biological function. However, the recent generation of mouse strains lacking MC-CPA has opened up new possibilities for investigations related to this protease. This recent development has revealed a role for MC-CPA in regulating innate immunity responses, including the degradation of harmful substances such as the vasoconstrictive factor endothelin 1 and snake venom toxins. Here, we summarize the current knowledge of MC-CPA.