Abstract: An intravenous-line flow-control system for receiving a disposable cassette disposed in the intravenous line. The system prevents the free flow of intravenous fluid through the cassette when the cassette is removed from the system. The system includes a control unit having a door or similar structure for holding the cassette against a receiving surface. The control unit includes a stepper motor and a transmission member for turning a rotatable control valve on the cassette in order to restrict variably the flow of fluid through the cassette. When the door is being opened but while the cassette is still held against the receiving surface, a motor-disengagement mechanism moves at least the transmission member, and preferably the stepper motor as well, from a first position, from which the control valve may be engaged by the transmission member, to a second position, in which the transmission member is disengaged from the control valve.