A medical fluid injector, replaceable syringe, and coupling mechanism to engage the syringe plunger to the plunger drive ram of the injector are provided wherein coupling involves a cam cleat-like mechanism, thereby allowing for cooperative movement between the plunger drive ram and syringe plunger. The coupling mechanism of the plunger drive ram is provided with first and second movable members, each having toothed, arcuate surfaces, while the coupling mechanism of the syringe plunger is provided with a knurled extension. Disengagement of the coupling mechanism is facilitated by translational movement of a face plate located on the forward end of the injector housing.