Abstract: The invention relates to materials which are beneficiating agent derivatives for bonding a beneficiating agent to filamentous keratins such as hair and wool and effect modification of the filamentous keratins by being chemically coupled thereto so as to become an integral part thereof. The materials for improving the properties of filamentous keratins comprise polypeptides having intact disulfide linkages and having an agent molecularly joined thereto through a bond independent of the disulfide linkages to form a polypeptide derivative. Such a derivative is chemically bonded to the filamentous keratins by a two-step process wherein disulfide linkages of both the derivatives and the filamentous keratins are split by the action of a reducing agent and disulfide linkages are then reformed by action of an oxidizing agent whereby at least some of the sulfhydryl groups formed by the action on the derivatives of the reducing agent are bonded to sulfhydryl groups of the filamentous keratins.