The present study was made to investigate the reparative process in an extraction wound and to clarify the difference between normal and infected conditions. The right and left 2nd molars of rats were extracted. As an experimental group, endotoxin (LPS of E. coli) was inoculated into the extraction sockets and the rats were sacrificed at intervals from 0 day to 2 weeks after extraction. The rats in control group were sacrificed at the same intervals. The submandibular lymph nodes and the extraction wounds were examined histologically and immunologically. In addition, quantitative studies of the macrophages and osteoclasts were also performed. The results obtained were as follows : 1. In the histological examination of the extraction wounds, intense round cell infiltration was observed in the alveolar bone marrow at 9 hours after extraction both of experimental groupand control group. In the alveolar crest and bone marrow of control group, a large number of osteoclasts appeared at 3 days after extraction. In experimental group, the osteoclasts appeared not only in the alveolar crest and bone marrows but in the base and middle walls of the extraction socket. 2. In the histological examination of the submandibular lymph nodes of the experimental group, numerous lymph nodules were observed at 1 day after extraction. The germinal centers were seen at the same time also. 3. In the immunological examination of the submandibular lymph nodes numerous IgG positive cells appeared in the inner cortex and medullary sinuses in experimental group at 3 days after extraction. Also the titers of serum antibody to LPS increased gradually to a maximum level in this period. 4. In the submandibular lymph nodes of control group, the number of macrophages reached a maximum level at 9 hours after extraction and decreased to a minimum between 9 and 12 hours. The results of experimental group were the same as those of control group except the number of macrophages were more numerous. 5. In the extraction wounds of control group, a large number of macrophages appeared at 9 hours and then increased rapidly. At 12 hours, the largest amount were shown and then decreased rapidly. The results of experimental group were also the same as those of control group except the cell number were more numerous. 6. In the extraction wounds of control group, the osteoclasts increased rapidly from 12 hours to 3 days, while in the experimental group the osteoclasts were more numerous and increased more rapidly as compared to the control group.