A tamperproof dispenser adapted for attachment to a squeeze tube dispenses a metered amount of material from the squeeze tube. The dispenser includes a body adapted for attachment to the squeeze tube, an inlet passage leading to a hollow valve seat, an outlet passage leading away from the valve seat, a rotor in the valve seat, a passage through the rotor adapted for collinear alignment with the inlet and outlet passages, and a slidable piston in the rotor passage. Material is dispensed by ejecting material from the squeeze tube which forces the piston to move through the rotor passage from adjacent the inlet passage toward the outlet passage. The rotor is rotated after each dispensing cycle to reverse the ends of the rotor passage for re-positioning the piston adjacent the inlet passage in preparation for the next dispensing cycle. The valve seat body has first and second annular peripheral edges surrounding adjacent opposite ends of the rotor. An annular shoulder on the rotor rotatably engages the first peripheral edge of the body. During assembly, a retainer plate is sonically welded to the rotor after the rotor is inserted in the valve seat, for rotatably engaging the second peripheral edge of the body. The annular shoulder and the retainer plate cooperate to retain and seal the rotor inside the valve seat in a tamperproof configuration which also ensures that the material will remain sterile during use.