The present invention involves, in some embodiments apparatus, such as dialysis apparatus, and methods for pumping fluid. Certain embodiments comprise a housing enclosing a pump drive assembly for actuating at least one pump on a removable fluid pumping cassette. The housing may comprise a door connected to the housing for enclosing an installed cassette in the housing upon closure of the door. The pump drive assembly may comprise a mating block configured for operative association with an installed cassette and may be configured to permit a sealing engagement with an installed cassette upon closure of the door. To facilitate sealing, certain embodiments of the door include a movable compression plate configured to exert a compressive force on an installed cassette against the mating block when the door is in a closed position.