A mixer cartridge for insertion into the basic body of a fitting contains a bottom making the water connection with the basic body of the fitting. This bottom is guided so as to be displaceable to some extent with respect to the cartridge housing. In the initial state, this bottom is positioned such that the seals present between it and the mixer system are compressed to an extent such that sufficient sealing-off takes place in the case of a low water pressure. It is arranged so as to be displaceable with respect to the cartridge housing in the direction of an increasing compression of these seals. Consequently, in the case of an increasing water pressure, a higher compression of the seals and therefore an increasing sealing action are made possible.