Abstract: A tamper-proof label adapted to be applied to joined surfaces of a member and heat sealed in place. A label includes a sheet adapted to bear identifying indicia with the sheet being cut in at least one location and the cuts being spaced so that they are located on each side of adjoining surfaces of the member to which the label is applied. The cuts permit material of the member to flow therethrough and over the exposed surface of the label when a predetermined amount of heat and pressure is applied so as to mechanically lock the label in position.