Exfoliated buccal epithelial cells have been widely used in microbial adherence studies, but present a number of problems due to their variable nature and to contamination with indigenous bacteria. An adherence assay was developed using HeLa cell monolayers which were washed with buffer, or treated with saliva or serum to mimic buccal or crevicular epithelial cells, respectively. A total of eighteen strains of oral bacteria tested showed a low affinity for untreated HeLa cells, but most strains adhered in high numbers to saliva treated HeLa cells. A few strains, usually present in the gingival crevice, demonstrated a high affinity for serum treated HeLa cells. Thus, salivary and crevicular fluid components appear to be specifically implicated in the selective adherence and colonization of bacteria on oral surfaces.