Ca²⁺-Activated neutral protease (CANP), that consists of 80K and 30K subunits, is converted to a 1ow-Ca²⁺-requiring form by autolysis in the presence of Ca Phosphatidylinositol greatly reduces the Ca²⁺-requirement for the autolysis of native CANP. However, this effect was not observed for CANP with a trimmed 30K subunit lacking the NH₂-terminal hydrophobic and glycine-rich region. This suggests that the NH₂-terminal hydrophobic region of the 30K subunit is important for the interaction of CANP with the cell membrane and that the calcium sensitivity of CANP is increased at the cell membrane through the effect of phosphatidyl inositol.