Although some studies suggest an anticaries effect of fluoridated bovine milk (F-milk) on enamel, evidence is still considered weak. Even more uncertain, the effect of F-milk on root caries remains largely unknown. This study evaluated the effect of F-milk on enamel and on root dentin demineralization using a validated Streptococcus mutans biofilm model, simulating a high cariogenic challenge. S. mutans (UA159) biofilms were formed on bovine enamel and root dentin saliva-coated slabs after measuring initial surface microhardness (SH). Biofilms were exposed to 10% sucrose 8x/day and treated 2x/day with either: (1) 0.9% NaCl (negative control), (2) bovine milk, (3) F-milk (5.0 ppm F as NaF) or (4) NaF 0.05% (anticaries-positive control). Medium pH was monitored twice/day, as a biofilm acidogenicity indicator. After 5 days for enamel and 4 days for dentin, biofilms were recovered to analyze: biomass, soluble proteins, viable microorganisms, and extra- and intracellular polysaccharides. Enamel and dentin demineralization were estimated by percentage of SH loss. Results were compared by ANOVA and Tukey's test. Neither acidogenicity nor biofilm composition differed among treatment groups in biofilms formed on enamel or dentin (p > 0.05). F-milk, however, significantly reduced enamel and dentin demineralization when compared with the negative control (p < 0.05). Also, F-milk was as efficient as 0.05% NaF to reduce enamel (p > 0.05), but not dentin demineralization (p < 0.05). These findings suggest that milk containing 5.0 ppm of fluoride is effective to control enamel caries and that it may be effective on root dentin caries prevention Ver menos