The invention relates to a door of a motor vehicle having an outer panel (1), an inner panel (2), a plate-shaped member (6), which is arranged on the door inside, for various add-on and mounting units, and reinforcement regions (14 to 17) for door functional elements and an installed stiffening member (3, 4) as a side-impact protector. In order to optimize the ratio of stiffness to side-impact protection of the door and to optimize the production expenditure of the door, the inner panel (2) and the stiffening member (3, 4) are formed as an integral frame-shaped component (5) which includes the reinforcement regions (14 to 17) from high-strength to superhigh strength steel sheet, and that are hot-worked. In order to be able to install the add-on and mounting units with sufficient mounting length, the plate-shaped member (6) is fastened to the inner panel (2) by means of a holding frame (7) which provides the desired mounting depth.