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By cDNA sequence analyses the proteases found within the secretory granules of immune/inflammatory cells appear to be translated initially as zymogens, but by amino-terminal sequencing they are stored within the granules in an active form. We now show that murine mast cell carboxypeptidase A (MC-CPA) is produced in a zymogen form (MC-pro-CPA) that is present at approximately 0.5% of the level of MC-CPA. MC-pro-CPA is an inactive precursor of MC-CPA and is located within the secretory granules of the mast cells. We have identified one mast cell line, KiSV-MC9, that produces MC-pro-CPA yet cannot process it to the active form despite the fact that these cells can process prochymase and protryptase to their active forms, indicating that a separate mechanism exists for activation of the serine proteases. We show that dipeptidylpeptidase-I is involved in the processing of murine mast cell prochymase and procathepsin G, but does not process MC-pro-CPA or protryptase. Thus, mast cell carboxypeptidase, tryptase, and chymase zymogens are each processed to their active forms by different mechanisms.