Surface-softened bovine enamel was submitted in vitro to a treatment with calcifying solutions containing calcium and phosphate (Ca/P ratio = 1.67). Variation in treatment was created by the addition to the solutions of magnesium, fluoride or both. Subsequently, the enamel samples were submitted to two successive softening steps. Alterations of the enamel surface due to the different treatments were evaluated with a hardness tester. No significant difference in the rehardening was observed between samples treated with a magnesium- or a fluoride-containing solution. A 2 X 2 factorial analysis of variance showed that both additions had a highly significant main effect on the rehardening. A significant interaction effect was obtained between magnesium and fluoride. Subsequent acid treatments revealed that only fluoride adequately protected the enamel surfaces from further dissolution. The presence of magnesium seemed to weaken the acid-protective action of fluoride in vitro.