A breast pump for pumping human breast milk has a flexible pumping membrane (6) and a stiff cap (5), which can be fitted onto said pumping membrane, this resulting in the formation of a pumping chamber (7) for generating a negative pressure. The cap (5) can be, or is, connected to a breast-shield unit (2) via a suction line (4). The pumping membrane (6) has an encircling collar (63) with a step (630) and a laterally projecting protrusion (61). The cap (5) has, on its inner side, an encircling abutment region (53) and also at least one form-fitting element (54), which is spaced apart from the encircling abutment region (53) and establishes a form fit (F) with the protrusion (61) of the pumping membrane. The cap (5) is positioned with a snap fit on the pumping membrane (6). In the abutment region (53), it establishes a joint frictional fit with a step (630) of the pumping membrane (6) and thus forms a seal (D). The snap-fit positioning of the cap (5) on the pumping membrane (6) is sufficient for the cap (5) to be retained with sealing action on the pumping membrane (6).
