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