The prime objective of this project is to find out the causes of bone loss in periodontal disease. Other pathological changes will also be investigated. 1. Germ-free rats will be monoinfected with organisms isolated from human cases of periodontal disease. previously it had been found that the mechanisms of bone loss were quite different with Gram-positive and Gram-negative organisms. We intend to try other organisms also isolated from human cases (especially Bacteroides melaninogenicus) to see what bone reactions occur. 2. Bacterial cell fractions isolated from human organisms have been injected into the gingivae of conventional rats. Cell walls were found to casue a typical lymphocyte response with destruction of osteoblasts. We intend to find out which constitutent of the wall, especially proteoglycan, is responsible for these changes. 3. To study absorption of bacterial products through the intact mucosa, silk sutures will be placed between the molar teeth of germ-free rats and will be soaked with products from human pathogenic organisms, such as endotoxin, proteoglycan and bacterial cytoplasm, the periodontium being examined histologically at intervals. BIBLIOGRAPHIC REFERENCES: J. T. Irving, M. G. Newman, S. S. Socransky and J. D. Heeley. Histological Changes in Experimental Periodontal Disease in Rats Mono-infected with a Gram-negative Organism. Archs. Oral Biol. 20, 219-220, 1975.