A method for monitoring a bus system having at least three users in a vehicle having a drive unit and a lockable vehicle interior; of the at least three users of the bus system, precisely one first user being formed as the superordinate user and initiating each data transmission on the bus system; at least one second user being formed as an element of a locking system in a vehicle, and at least one third user being arranged outside of the lockable vehicle interior, wherein the first user monitors the data transmission such that in at least one specific operating state of the vehicle and/or the bus system, the first user, for each data transmission on the bus system that was not initiated by it, introduces measures to prevent transmission of the data.