Cylindrical blocks of enamel with initial caries lesions were brushed twice a day for 6 days using amine fluoride or monofluorophosphate children's toothpaste (250 ppm F-). Some of the enamel samples were left in the oral cavity for three further days after the end of the trial, during which time a fluoride-free toothpaste was used for daily oral hygiene. Half of the enamel blocks showed plaque formation during this period. The other half were kept clean. Fluoride soluble in KOH was only detected on the enamel blocks that had been brushed with amine fluoride children's toothpaste. The amount of solidly bound fluoride was significantly higher following the use of amine fluoride children's toothpaste than with monofluorophosphate children's toothpaste. The amounts of KOH-soluble fluoride and solidly bound fluoride fell again 3 days after discontinuation of fluoride application. But only the drop in the amount of KOH-soluble fluoride in demineralised tooth enamel after application of the two children's toothpastes can be explained by the differing reaction mechanisms of the two fluoride compounds.