Abstract: A tamper-proof cap for containers, bottles and the like comprising a first driven member and a second drive member concentrically mounted on the first member and having a window through which a visible signal may be observed by a viewer. A rupturable capsule containing signal media is supported on the first driven member and displays a signal of first characteristic through the window. Upon opening the cap by causing the drive member to engage the driven member, the capsule ruptures causing the signal media to display a signal of a second characteristic through the window indicative of the fact that an attempt had been made to open the cap. In an alternative embodiment, a lens mounted on the window portion of the drive member enables viewing of the signal characteristic when the containers bearing the caps of the invention are stored on a relatively high shelf.