Abstract: A security lock for doors which may be unlocked from one side of the door only, and comprising sliding bar or pin means selectively engagable with the oppositely disposed door frames for locking the door against entry therethrough by unauthorized personnel. Knob means accessible from one side of the door only is operably connected with the bar means for positively locking thereof in the engaged position with the door frame, and quick relase means is provided for cooperation with the knob means for release of the locking engagement of the bar means with the door frames in order to permit passage through the door under panic conditions, or the like.