An actuating mechanism for a hypodermic syringe comprises a flexible tongue (48) allowing the actuating mechanism to be bent to receive a cartridge (10) of injectant closed at its forward end with a septum (14) and at its rearward end with a slidable bung (18). The forward end of the cartridge (10) is fitted into a socket (36) wherein the tail (34a) of a hypodermic needle (34) pierces the septum (14) to access the injectant. With the cartridge (10) fitted, the tongue (48) is straightened, and a spring clip (50) holds the cartridge (10) in position. A plunger (38) can then be depressed, though a guide (42), to drive the bung (18) forward and make the injection. The cartridge (10), which may be a conventional cartridge of any kind, keeps the assembly rigid in use, and the whole assembly is disposed of after a single use, without dismantling.