In this study fluoride was added to an artificial caries system in vitro. Both the fluoride concentration and the pH of the gel system had an effect on subsequent lesion formation. At pH 4.0 the addition of fluoride to a concentration of 10 ppm did not inhibit lesion formation. Gross loss of surface tissue was noted beneath which laminated lesions were formed with histological features characteristic of remineralization deep within the lesion. However, at pH 4.5 the presence of fluoride at the same concentration (10 ppm) resulted in either partial or complete inhibition of lesion formation.