The soluble β-chymases mouse mast cell protease-1 (mMCP-1) and rat mast cell protease-II are predominantly expressed by intestinal mucosal mast cells (IMMCs) and may promote mucosal epithelial permeability when released during intestinal allergic hypersensitivity responses. To study the function of these chymases, we generated mice with a homozygous null mutation of the mMCP-1 gene and investigated their response to infection with the intestinal nematode Nippostrongylus brasiliensis. Whereas mMCP-2, -4, and -5 were transcribed normally, there was no transcription of the mMCP-1 gene in null (−/−) mice, nor was mature mMCP-1 protein detected in (−/−) jejunal mucosa. In contrast, levels of mMCP-1 in wild-type (+/+) jejunal mucosa increased 200- to 350-fold from 0.66 μg mMCP-1/g wet weight in uninfected mice to 129 and 229 μg/g wet weight on days 8 and 10 of infection, respectively. The kinetics of IMMC recruitment differed in −/− mice compared with +/+ controls on days 8 (P < 0.05) and 10 (P < 0.03) of infection. The IMMCs in infected −/− mice stained poorly, if at all, for esterase with naphthol AS-D chloroacetate compared with the intense staining observed in +/+ controls. Ultrastructurally, the prominent crystal intragranular structures that are found in intraepithelial +/+ IMMCs were absent from −/− IMMCs. These data show that disruption of themMCP-1 gene leads to profound histochemical and ultrastructural changes in IMMC granules.