Abstract: An alarm system for a cargo box made up of a plurality of panels adapted to be assembled to form the box, in which each panel is provided with a conductor distributed throughout the panel, and in which the conductors of adjacent panels are connected in series when the box is assembled to complete a circuit including an alarm device when a lock is operated. The arrangement is such that the alarm system is silent so long as the conductors complete the circuit. When the circuit is broken with the lock still locked, the alarm system gives a signal indicating that an unauthorized person is attempting either to open the container or to break through a panel thereof in an effort to gain access to the contents.