The source of fluoride ions (F––) in sodium monofluorophosphate (MFP) dentifrices and their importance in the fluoridation of enamel relative to MFP anions has been investigated. Commercial samples of MFP used by dentifrice manufacturers contained 1.3–3.2% NaF, indicating that some fluoride is incorporated during manufacture. No MFP was found in powdered bovine enamel after exposure to dentifrice slurries; neither was there any indication that MFP could be a source of fluoride in the mouth. On the other hand, uptake of F by enamel was related to the initial fluoride concentration of the dentifrices rather than to the MFP concentration and uptake could usually be correlated with the disappearance of fluoride from the slurries. Tracer experiments in which either fluoride or MFP was labelled with 18F showed conclusively that 92–97% of the F taken up by enamel arose from the fluoride component of the dentifrices. It is usually considered that the caries-reducing action of topically applied F, whether mediated through changes in enamel solubility or plaque metabolism or both, depends on an initial uptake by enamel. If the established clinical effect of MFP dentifrices also depends on the incorporation of F into enamel, then MFP itself is probably not the active agent in these dentifrices.