The in vitro incorporation of non-KOH extractable fluoride into human enamel during a topical application of potassium fluoride and after 24 and 96 h incubation was studied with and without surface-active agents (SAA) in the solutions. The SAA included Lodyne S-110, Zonyl FSC, Zwittergent 310 and 316, dodecylamine-HCl and sodium dodecyl sulfate. Incorporation of Lodyne S-110 and Zonyl FSC into the fluoride solutions effected increased deposition of fluoride in enamel already during the application. All SAA effected increased deposition of fluoride in specimens incubated for 24 h after application. Presence of SAA did not enhance fluoride deposition in specimens incubated for 96 h after application. The findings are consistent with the view that the presence of SAA in fluoride solutions for topical therapy can increase the deposition of firmly bound fluoride in the enamel and that this increase is achieved by affecting the kinetics of enamel fluoride interactions.