The colonization of Actinomyces viscosus strain Ny-1R on the molar teeth of conventional and ex-germfree rats of various ages fed either a high-sucrose diet, a high-glucose diet, or laboratory chow was studied. Conventional rats directly after weaning and up to 30 days of age are less susceptible to experimental infection by strain Ny-1R than are older rats regardless of the test diet. The relationship between host age and susceptibility to infection is also demonstrable in ex-germfree rats fed a high-sucrose diet. Host factors responsible for the differences in susceptibility were investigated. The results from these studies do not implicate host antibodies, host indigenous flora, or host saliva. In other studies, it was demonstrated that within the mouths of rats, strain Ny-iR preferentially colonizes in the pits and fissures of the molar teeth rather than on the dorsum of the tongue or on the vestibular mucosa. In short-term experiments, it was found that strain Ny-1R attaches to the first molars of 40-day-old conventional rats to a greater extent than it attaches to the first molars of 20-day-old rats. The differences in attachment and subsequent colonization of strain Ny-iR in 20-and 40-day-old rats may be related to the varying amounts of the reduced enamel epithelium and connective tissue present in the fissures of the molar teeth. Actinomyces spp. are among the predominant organisms in human dental plaque (1, 18, 26) and have been implicated in the etiology of certain types of periodontal disease (20, 21, 26, 29, 30), root surface caries (19, 31, 32), and fissure caries (16). However, little is known about the factors that govern their establishment in the mouth. Information about the mechanisms involved in the colonization of Actinomyces spp. on oral surfaces or the susceptibility of a host to infection by these organisms may aid in the prevention and eradication of oral infections by Actinomyces spp. Recent studies suggest that Sprague-Dawley rats up to the age of about 3 months are more susceptible to infection by strains of Streptococcus mutans than are older rats (36). These observations with S. mutans, presently considered as an important causative agent in dental caries (15, 34), prompted similar investigation of the role of host age in the susceptibility of rats to infection by Actinomyces viscosus. MATERIALS AND METHODS Animals and diet. Conventional male Sprague-Dawley rats of various ages were obtained from Charles River Breeding Laboratories, Wilmington, Mass. Mothers and male pups were caged together until weaning of the pups at 17 to 21 days of age. All rats were individually housed in screen-bottomed stainless steel cages without bedding and were provided with experimental diet and drinking water ad libitum. The experimental diet consisted of 34.7% vitamin-free casein, 5% salt mixture W, and 3.5% brewers' yeast extract (all from Nutritional Biochemicals Corp., Cleveland, Ohio); this was supplemented with either 56% sucrose or 56% glucose. In some experiments, pelleted laboratory chow (Ralst...