The effect of purified monofl·uorophosphate (MFP) on the formation of hydroxyapatite (HA) in supersaturated calcium phosphate solutions was determined. In solutions initially at pH 7.4 and seeded with HA crystals, MFP was hydrolysed to a small extent, releasing F^––. Once the crystal growth-enhancing property of this F^–– was compensated for, 4 mmol/l MFP could be shown to inhibit precipitation by 40%. Without compensation for F^––, MFP appeared to inhibit precipitation by only 18%. This inhibition was weaker than that caused by pyrophosphate or ethane-1-hydroxy-11-diphosphonic acid. Because MFP is hydrolysed on apatite surfaces to F^––, use of sodium MFP as an anticaries agent is unlikely to cause significant inhibition of enamel remineralization.