We previously reported that mast cell α-chymase cleaves and activates progelatinase B (progel B). Outside of cells, progel B is complexed with tissue inhibitor of metalloproteinase (TIMP)-1, which hinders zymogen activation and inhibits activity of mature forms. The current work demonstrates that dog BR mastocytoma cells, HMC-1 cells, and murine bone marrow-derived mast cells secrete TIMP-1 whose electrophoretic profile in supernatants suggests degranulation-dependent proteolysis. α-Chymase cleaves uncomplexed TIMP-1, reducing its ability to inhibit gel B, whereas tryptase has no effect. Sequencing of TIMP-1’s α-chymase-mediated cleavage products reveals hydrolysis at Phe12-Cys13 and Phe23-Val24 in loop 1 and Phe101-Val102 and Trp105-Asn106 in loop 3 of the NH2-terminal domain. TIMP-1 in a ternary complex with progel B and neutrophil gelatinase-associated lipocalin is also susceptible to α-chymase cleavage, yielding products like those resulting from processing of free TIMP-1. Thus, α-chymase cleaves free and gel B-bound TIMP-1. Incubation of the progel B-TIMP-1-neutrophil gelatinase-associated lipocalin complex with α-chymase increases gel B activity 2- to 5-fold, suggesting that α-chymase activates progel B whether it exists as free monomer or as a complex with TIMP-1. Furthermore, inhibition of α-chymase blocks degranulation-induced TIMP-1 processing (absent in α-chymase-deficient HMC-1 cells). Purified α-chymase processes TIMP-1 in BR supernatants, generating products like those induced by degranulation. In summary, these results suggest that controlled exocytosis of mast cell α-chymase activates progel B even in the presence of TIMP-1. This is the first identification of a protease that overcomes inhibition by bound TIMP-1 to activate progel B without involvement of other proteases.