Release of chymotrypsin-like esterase from isolated rat peritoneal mast cells by arginine-rich and lysine-rich calf thymus histones was investigated. The arginine-rich histone released the esterase, but the lysine-rich histone did not release the esterase from the mast cells. The isolated mast cells spontaneously released the esterase gradually. The chymotrypsin-like esterase released from the mast cells is considered to digest the destructed material in tissues in a state of chronic destruction, such as chronically inflamed tissues and malignant tumours.