The present experiments showed that fluoride and monofluorophosphate in low concentrations could desorb albumin and salivary glycoprotein adsorbed to hydroxyapatite in vitro. It is suggested that the principle involved is competitive inhibition, the fluoride and monofluorophosphate ions replacing acidic protein groups adsorbed to calcium ‘sites’ on the mineral surface. Bacteria adsorbed to hydroxyapatite could be desorbed in the same way indicating a similar mechanism. The adsorption of biological material to teeth seems to be strongly dependent on the surface chemistry of the teeth. Desorption of pellicle protein and bacteria may be one of the mechanisms by which fluoride and monofluorophosphate exert their cariostatic effect in vivo.