Human strains of Actinomyces viscosus and A. naeslundii differ in the time of their appearance and in their patterns of colonization in the mouth. Strains of these organisms were found to differ in their abilities to adsorb to saliva-treated hydroxyapatite (S-HA) surfaces, thought to mimic the teeth, and these differences parallel their patterns of colonizing the dentition. Thus, strains of A. viscosus tended to adsorb in higher numbers to hydroxyapatite (HA) treated with saliva of older children and adults than with saliva of younger children (ages 6 to 11). These salivary changes may account for the increased frequency with which this organism can be isolated from the mouths of children as they grow older. In contrast, strains of A. naeslundii and Streptococcus mutans did not show a preference for attaching to either type of S-HA. Strains of A. viscosus also generally adsorbed in higher numbers than A. naeslundii to HA treated with adult saliva; this may explain why higher proportions of A. viscosus are usually recoverable from the teeth of adults, even though A. naeslundii is generally present in higher proportions in saliva. Significant variation was noted between strains and between saliva samples collected from different donors. The differences in adsorptive behavior of strains of these species suggests that they are binding to different receptors in the salivary glycoprotein coating on HA surfaces. Adsorption of A. naeslundii ATCC 12104 was enhanced when S-HA was pretreated with neuraminidase, but this had little effect upon the adsorption of other Actinomyces strains tested. Adsorption of strain ATCC 12104 to S-HA was also strongly inhibited by fructose and sucrose and weakly inhibited by glucose, maltose, galactose, and lactose. However, other strains of A. naeslundii tested were affected less, or not at all, by these sugars. Adsorption of two strains of A. viscosus was not affected by any of the sugars or amines tested.