Abstract: A security lock box includes a receptacle having an access opening and a hinged door. A vertical door edge adjacent th hinge is provided with an edge return which engages an inwardly curved lip along the receptacle opening for preventing entry through removal of a hinge pin. The door additionally has flanges extending transversely along top and bottom edges. The flanges are adapted for cooperative interfitting within confronting guard recesses along the margin of the receptacle opening. A locking mechanism having a pair of selectively slidably locking bars is positioned within the access opening. The locking bars are extendable through slots within the door flanges for securing the door in a locked mode. The locking mechanism is activated by a cylinder lock mounted in the door. The cylinder lock includes a cylindrical housing for accommodating a removable key plug and operating key adapted for interchangeable applications within a plurality of similar lock cylinders.