The anticaries effect of professional fluoride (F) application has been attributed to calcium-fluoride-like deposits (CaF2) formed on enamel, but this has not been clearly demonstrated. We hypothesized that CaF2 formed on plaque-free enamel by F application would reduce enamel demineralization due to the increase of F availability in fluid of subsequently formed plaque. We created distinct levels of CaF2 on enamel to evaluate a dose-response effect. Enamel blocks were mounted in contact with a S. mutans test plaque and used in situ by 10 volunteers. F released to the fluid phase of this substrate (“plaque fluid”) was measured before a cariogenic challenge. “Plaque fluid” F concentration was highly correlated to the enamel CaF2 concentration (r = 0.96, p < 0.001) and to consequent enamel demineralization (r = −0.75, p < 0.001). The results suggest that F released to plaque fluid from CaF2 formed on enamel may play a significant role in the anticaries effect of professionally applied F agents Ver menos