Abstract: A seal for container locks and the like comprises two units (1,2,3;8,14,15) which are snapped into each other. The snap action is carried out by means of a plug (14) forming part of one of the units (8,14,15) and which normally closes a cavity (13) containing an indicating means (21). When the parts are separated the plug is drawn out from this position and allows the indicating means to leave said cavity. In one embodiment the plug (14) is hollow and is closed by means of a steel pin (15) which when inserted has put the indicating means under pressure and the plug has blocking vanes (18). In another embodiment the plug is not hollow but has a weak lower portion and supports a resilient blocking ring. A steel pin floats in the cavity and can break the weak wall when an attempt to brake the seal is made.