Abstract: An I.V. pump cassette includes an enclosed pumping chamber with an inlet and outlet adapted to be connected to I.V. tubing. A reciprocable plunger extends from the top of the pumping chamber. The cassette is arranged to be received in a semi-cylindrical chamber in a driver mechanism wherein a horizontally extending support portion of the cassette can be supported in a semi-circular recess and clamped thereon by overlying clamp members of the driver mechanism. An upstanding semi-circular fence is provided at the front end of the cassette to activate a laterally pivotable switch in the appropriate driver mechanism and to create an interference with the wall of the semi-cylindrical chamber in an inappropriate driver mechanism.