The relationship of neuraminidase-sensitive receptors and putative hydrophobic interactions to high- and low-affinity binding sites in experimental salivary pellicles for Streptococcus sanguis C5 was investigated. NaSCN, an inhibitor of hydrophobic interactions, reduced the number of cells which adsorbed to pellicles to a greater extent than NaCl or KCl when both low and high streptococcal concentrations were used in assays. However, NaSCN was not more effective than NaCl or KCl in desorbing 3H-labeled salivary pellicle components from hydroxyapatite, and NaSCN pretreatment of either strain C5 cells or the salivary pellicles did not destroy or remove either the streptococcal adhesions or the pellicle receptors. Neuraminidase treatment of pellicles or the presence of sialic acid-containing gangliosides only inhibited S. sanguis adsorption when low streptococcal concentrations were used. At these concentrations, S. sanguis adsorbs primarily to high-affinity pellicle binding sites. Adsorption isotherms indicated that neuraminidase-sensitive interactions were mainly responsible for the high affinity of these binding sites, whereas putative hydrophobic interactions inhibitable by NaSCN were mainly associated with the numbers of binding sites available. Sugar inhibition studies suggested that the two classes of binding sites previously demonstrated in untreated salivary pellicles for S. sanguis C5 are not the result of a partial conversion of high-affinity sites to low-affinity sites due to removal of sialic acid residues.