The invention relates to a method and a device for the sterile filling of a disposable injector with a defined dose of an injection solution, wherein the disposable injector comprises a cylinder/piston unit with a cylinder and with a piston that can be pulled back manually. The defined dose of the injection solution is stored in an interior of a container of a needle syringe held in a bored stopper. The cylinder interior is connected to the container interior in a gas-tight and liquid-tight manner through the bored stopper. The cylinder/piston unit is filled by pulling back the piston or by pressing the needle syringe piston. With the present invention, a method and a device for filling a disposable injector are developed in which, without intermediate weighing or risk of inaccurate readings, the disposable injector can be filled with a defined dose of an injection solution from a syringe with a fused-in needle.