A modular filter system using replaceable filter cartridges includes a locking arrangement that prevents rotation of the flow control valve spindle in the filter head, but includes an unlocking arrangement on the cartridge that unlocks the locking arrangement if the proper replacement cartridge is used. The locking arrangement may include spring biased locking pins carried on the filter head and matching unlocking pins on the filter cartridge to operate through an intermediate adaptor ring that is set on assembly to match the positions of the locking and unlocking pins.