We performed immunohistochemical and histopathological investigation on the inflamed gingiva from the patient with Yusho and on the periodontal tissue from PCB poisoned rats respectively. The results obtained were as follows. 1) In the gingiva from the Yusho patients, the proportions of plasma cells, B cells, T cells in the inflammatory cells and CD4/CD8 ratio were 47.9%, 18.3%, 10.7%, and 2.84, respectively. 2) In the periodontal tissue from rats with experimental periodontitis, we found no definitive difference between the PCB poisoned rats and the clinically healthy rats.