Seven alkyl phosphate derivatives and three nonionic surfactants were examined for their ability to inhibit the adherence of 3H-labelled cells of Streptococcus mutans to hydroxyapatite (HA) treated with buffer (BHA) or parotid saliva (SHA). None of the compounds alone hindered binding of bacteria to BHA or SHA effectively. A combination of certain of the alkyl phosphates, notably a disodium phosphate of 1-octadecanol, and nonionic surfactant at a 1:1 molar ratio gave a strong inhibition of S. mutans adherence. Treatment of SHA with this combination resulted in 98% reduction of adherence. Adsorption of the two types of surface-active agents alone and in combinations was studied using 14C-labelled agents. Electrophoretic measurements revealed that surfaces treated with only alkyl phosphate became highly negatively charged, while surfaces treated with the combination had a ζ-potential of around zero. The results suggest that treatment with surfactants might be used to interfere with bacterial adherence on the tooth surface