Calcium-activated neutral protease (CANP), a typical intracellular protease, is composed of a catalytic 80K subunit (80K) and a 30K subunit (30K) of unknown function. The structure of rabbit CANP 30K was examined to clarify its role in the enzyme function. It has a clear two-domain structure composed of 266 amino acid residues. The N-terminal domain presumably determines the location of CANP, whereas the C-terminal domain is a calmodulin-like Ca2+-binding domain and regulates CANP activity.