The continuous and/or improper ingestion of irritants, including alcohol, NSAIDs, and Helicobacter pylori, often leads to serious gastropathies, affecting a wide range of people. A complex gastric defense system works to protect against these threats, for example by secreting mucus. Recently, by analysis of gene targeting mice for two gastrointestinal-tract-specific calpains, calpain-8 and calpain-9, we have demonstrated that they are cooperatively involved in the mucosal defense against stress-induced gastropathies. Calpains-8 and -9 are members of Ca2+ -dependent intracellular proteases comprising a superfamily in almost all eukaryotes, and form a functional complex, "G-calpain", expressed specifically in the mucus-producing cells. In this review, we show our recent results on calpains -8 and -9, and discuss gastric mucosal defense mechanisms involving them.