A dialyzer header cleaning device composed of: a connecting member for connecting the device to the header, the connecting member having a longitudinal passage with a longitudinal axis; and a flow directing member having first and second opposed ends and a fluid passage extending between a fluid inlet at the first end and a fluid outlet at the second end. The flow directing member has a portion that extends from the second end toward the first end and is held in the longitudinal passage. The flow directing member is manually rotatable about the longitudinal axis relative to the connecting member and is manually movable along the longitudinal axis relative to the connecting member. The fluid outlet of the flow directing member is constructed to eject at least one liquid stream in a direction transverse to the longitudinal axis when fluid is supplied to the fluid inlet.