The aim was to determine the fluoride retention in plaque-covered and clean incipient enamel lesions after topical application of a CaF2/NaF varnish (Bifluorid 12). In 50 specimens of bovine enamel an incipient lesion was produced with acidic hydroxyethylcellulose (pH 4.8; 72 h). 40 specimens were fluoride varnished; the 10 remaining specimens were used for measuring baseline fluoride content. Each six of the fluoridated specimens were recessed in the buccal aspects of an intraoral appliance worn for 5 days. During the experimental period one side of the appliance was kept clean, and plaque growth was allowed on the other. KOH-soluble and structurally bound fluoride were determined immediately, 1 day, 3 days and 5 days after fluoridation, and compared with the baseline fluoride content of the enamel. Immediately after fluoridation, a considerable amount of KOH-soluble fluoride was bound, but after 5 days 80% had been lost. Simultaneously a significant increase of non-KOH-soluble or structurally bound fluoride was detected in both plaque-covered and clean enamel. It is evident that this CaF2/NaF varnish deposits more KOH-soluble fluoride on the surface of demineralized enamel than other varnishes, but after 5 days fluoride retention is similar to that from other varnishes.