The rodent has been used for a number of years as a model system for the production of human dental caries. (1) the rat has provided to be a suitable model since caries can be readily induced, and the results are in general reproducible (1,2,3). Valuable insights into the cariogenicity of individual bacterial species has been obtained (4,5). More recently the role of host immune factors in the modulation and control of dental caries has been extensively studied in the rat and in the hamster (2,3) and have revealed that either oral or submucosal immunization may protect against caries (2,3,4). In spite of the wealth of data in this field, little is known about the bacterial interrelationships in relation to bacterial colonization and dental caries. In the in vitro model of caries important differences in the production of caries have been found when more than one species of bacteria is used in the model (Heilman and Clarkson). It has been shown that when S. mutans and A. viscossus are fermented with sucrose in the presence of teeth, subsurface lesions in enamel developed with S. mutans and much smaller or no lesions developed with A. viscossus. As no pH differences were detected (both were at pH 4.5) and the teeth were suspended in the media for the same length of time, these results were surprising. However, in the same system both bacteria produced comparable lesions on the root surface. Further investigations are planned to: mix the bacterial strains; to monoinnoculate with S. mutans and then innoculate with the A. Viscossus (to reverse this); to vary the fermation conditions; and to introduce immunological and nonimmunological factors into the system. We wish to extend the in vitro work into a suitable model in vivo, and to be able to study the effects of saliva and host immunological factors on the colonization of bacteria, and on the subsequent development of caries. The rat was selected since this is currently the most well studied animal model and has been shown to be reproducible. For the study of individual bacterial species it is necessary to use a gntobiotic unit.