Tooth movement, the phenomena and mechanisms of which are still contoroversial, can be considered as part of the result of the inflammatory processes. The purpose of this study was to examine the activity of macrophage and T-cell, playing important roles in the immune reaction, in the periodontal ligament of dog, in which experimental tooth movement was performed. Six one and half year-old dogs, a control and 5 experimentals, were studied. Light force (50-75g) was applied by placing open-coil spring between left mandibular premolars ; heavy force (250-300g), between right mandibular premolars. Experimental dogs were sacrificed at 12 hours, 1, 3, 7 and 14 days since force application, respectively. And the histologic and the immunhistochemical evaluation on the obtained tissue were performed, using a-1-antichymotrypsin and CD3 antibodies. The results were as follows : 1. There were more inflammatory cell infiltrations in heavy force group than in light forcce group until 3 days. But from 7 days on, no difference was not observed between groups ; Such an infiltration was more evident at pressure side than at tension side. 2. Osteoclastic activity at pressure side began to be seen in 12 hours, increasing until 7 days. After then it decreased ; Such an activity was more evident in heavy force group than in light force group. 3. Tearing of periodontal ligament and vascular dilatation at tension side began to be seen in 12 hours, increasing until 3 days. After then it decreased ; Such an observation was more evident in heavy force group than in light force group, but there was no difference between groups in 14 days. 4. a-1-antichymotrypsin expression in control group was positive, mainly in sulcular epithelium, but negative in periodontal membrane, pulp, bone cells. 5. a-1-antichymotrypsin expression in experimental group was more positive in pressure side than in tension side ; The expression was a little more positive in cervical area of tooth until 3 days, but after 7 days, it was more positive in apical area. 6. a-1-antichymotrypsin expression in light force group began to be observed in 12 hours and reached to the greatest level in 7 days, after which it decreased ; In heavy force group, it was the greatest in 3 days, after which it decreased. 3 Expression in the periodontium was almost negative.