Abstract: A mullion assembly mountable in a door frame includes a bottom fitting attached to a floor and a top fitting attached to the door frame above the bottom fitting. The top fitting supports a bolt engagement assembly for engaging a lock assembly positioned in a mullion cavity of a mullion. The lock assembly has a bolt extendable from the mullion cavity to engage the bolt engagement assembly supported by the top fitting and hold the mullion in a fixed position. Retraction of the bolt is controlled by a key lock assembly that allows controlled access disengagement of the bolt from the bolt engagement assembly, followed by pivoting movement of the mullion away from its upright position in the door frame.