Mucosal mast cells are in the mouse predominantly found in the epithelium of the gastrointestinal tract. They express the beta-chymases mMCP-1 and mMCP-2. During nematode infections these intraepithelial mast cells increase in numbers and high amounts of mMCP-1 appear in the jejunal lumen and in the circulation. A targeted deletion of this enzyme leads to decreased ability to expel the intraepithelial nematode Trichinella spiralis. A suggested role for mMCP-1 is alteration of epithelial permeability by direct or indirect degradation of epithelial and endothelial targets, however, no such substrates have yet been identified. To enable a screening for natural substrates we performed a detailed analysis of the extended cleavage specificity of mMCP-1, using substrate phage display technology. In positions P1 and P1' distinct preferences for Phe and Ser, respectively, were observed. In position P2 a high selectivity for large hydrophobic amino acids Phe, Trp and Leu was detected, and in position P2' aliphatic amino acids Leu, Val and Ala was preferred. In positions P3 and P4, N-terminal of the cleaved bond, mMCP-1 showed specificity for aliphatic amino acids. The high selectivity in the P2, P1, P1' and P2' positions indicate that mMCP-1 has a relatively narrow set of in vivo substrates. The consensus sequence was used to screen the mouse protein database for potential substrates. A number of mouse extracellular or membrane proteins were identified and cell adhesion and connective tissue components were a dominating subfamily. This information, including the exact position of potential cleavage sites, can now be used in a more focused screening to identify which of these target molecules is/are responsible for the increased intestinal permeability observed in parasite infected mice.