Abstract: The present invention relates to a lock cover device particularly intended for use with so-called mortise locks. The device is fabricated of resilient polymeric material which may be readily mounted over the mortise lock, normally without the requirement for completely removing the lock from its mounted position and is characterized by the incorporation of a movable shutter member selectively shiftable between a stable sealing position and a stable keyway exposing position, the assembly including resilient cam and follower means for selectively biasing the shutter to one or the other of said stable positions, the shutter being preferably unstable at any intermediate position.