A liquid-liquid mixing device (10, 210) includes a barrel (20, 220) with a liquid port (23) at or adjacent one end. A plunger assembly (30) is reciprocably moveable along an axis in the barrel (20, 220) and includes a seal member (31, 231) and an agitator (50, 250). The seal member (31, 231) is in sealingly slidable engagement with the internal wall of the barrel (20, 220) to define a variable volume chamber (24, 224) therein in communication with the liquid port. The agitator (50, 250) is reciprocably moveable in the variable volume chamber (24, 224), which agitator (50, 250) includes one or more end to end passages (54) through which liquid in the chamber (24, 224) is forced as the agitator (250) reciprocates in the chamber (24, 224). The device (10) also includes a mode selector mechanism (60, 28, 46, 64, 65, 90, 92, 94, 96) for selection between at least two modes of operation for the plunger assembly, wherein the mode selector mechanism (60, 28, 46, 64, 65, 90, 92, 94, 96) is adjustable between two or more modes whereby movement of the plunger assembly (30) effects either movement of the agitator (50, 250) with the seal member (31, 231) or movement of the agitator (50, 250) relative to the seal member (31, 231), depending on the selected mode.