mMCP-8 and its rat homologues, rMCP-8, -9, and -10, form a new group of mast cell/basophil proteases, which are more closely related to the T-cell granzymes and neutrophil cathepsin G than to the mast cell tryptases and chymases. mMCP-8 showed a high degree of homology with mouse granzyme B in the critical regions for determining substrate cleavage specificity. It preferentially cleaves after Asp residues. mMCP-8 mRNA is highly expressed in a mouse connective tissue MC-like tumor line. However, it could not be detected in mouse liver, intestine, lung or ears, indicating very low expression in normal tissues. A strong increase in mMCP-8 levels in the lungs can be detected in infected animals.