The affinity for hydroxyapatite of salivary factors inducing aggregation of some strains of Streptococcus mutans, Streptococcus sanguis and Streptococcus mitis was studied in vitro using unstimulated parotid saliva and a synthetic hydroxyapatite. Large variations were demonstrated in the affinities but the factor for strains within each serotype of S. mutans showed similar adsorption isotherms and required equal amounts of apatite for complete adsorption. This supports earlier findings of a high specificity for the aggregation-inducing substances. The factors for S. sanguis strains and the serotype d strains of S. mutans, recognized as members of the early plaque, showed the strongest affinity for apatite.