Adsorption of bacteria to saliva-coated surfaces, saliva-induced aggregation of bacteria and interactions between different genera of oral bacteria (coaggregation, interbacterial adherence) are considered important in the formation and persistence of dental plaque. Studies have been carried out to define the galactoside specificity in the lectin reaction specific for the coaggregation between Streptococcus sanguis and Propionibacterium acnes. Physiochemical interactions important for this coaggregation have also been defined. Experiments to isolate and/or to determine stability of cell surface coaggregation factors have yielded results consistent with a sensitive lectin-protein on the S. sanguis and a stable carbohydrate-containing receptor on the P. acnes. Growth of Actinomyces viscosus and Streptococcus sanguis strains in sublethal concentrations of antibiotics has resulted in the production of variants with altered ability to undergo coaggregations and/or to adsorb to saliva-coated hydroxyapatite. Studies are in progress to characterize antibiotic induced surface-structure changes at the molecular level. Parotid salivas from rats treated with Beta-adrenergic antagonist or Beta-adrenergic agonist show different potentials to support bacterial aggregation and adsorption of bacteria to hydroxyapatite. These differences have been associated with biochemical changes in saliva composition.