The objectives are to explore the role of various parameters of the host parasite relationship pertaining to chronic human periodontitis. The major emphasis is to define the role of microorganisms in periodontal disease and to delineate the host factors mediating the disease. Particular emphasis is placed on the role of hypersensitivity as the mechanism in periodontal disease and how specific strains of microorganisms are involved in the etiology of the disease. The rice rat demonstrates the same type of tissue damage in its gingival tissue as is seen in humans with periodontal disease. To study the effect of particular microorganisms, the rice rat will be put in a gnotobiotic state, either through the use of various antibiotics which would eradicate most of the microorganisms found in the normal animals or by putting the animals under germfree conditions. In the latter case the specific effect of a single strain of microorganisms could be studied more carefully than could be accomplished under gnotobiotic or conventional conditions. It is hoped that not only will knowledge be obtained with respect to the interplay among the different etiological factors but also with regard to regimens that could be used in both preventive and/or therapeutic procedures.