The cariostasis of a mouthrinse containing 100-ppm fluoride versus a vehicle control was evaluated against experimental root caries on human and bovine cementum mounted on prosthetic devices. Subjects swished 10 ml of mouthrinse for 60 s twice daily and immersed their appliances with the test specimens into 0.1 M lactic acid buffer (pH 4.0) for 10 min three times daily. Recovered specimens were submitted to fluoride biopsies or to an acid test in vitro. The 100-ppm fluoride mouthrinse contributed to increased fluoride incorporation into the experimental lesions. The microradiographic lesions showed a thicker layer of acid-resistant mineral at the outer layer of specimens exposed to the fluoride mouthrinse than on those exposed to the vehicle control.