Antilymphocyte sera(ALS) were raised against New Zealand white Rabbit with tonsil(ATS) and spleen(ASS) of adult dogs. Ten adult dogs were divided into 4 groups : In group 1, ATS was injected into three dogs ; in group 2, ASS was injected into two dogs ; in group 3, ATS with ASS was injected into three dogs; and in group 4, normal rabbit serum was injected into two dogs. Twelve hours after the injection of ALS, the peripheral blood finding, bone marrow picture and histologic finding were examined and compared with the findings before the injection. The results were as follows : 1. Peripheral blood : Mean percentage decreased of lymphocytes decreased to : 52.7% in group 1; 69.2% in group 2; 69.7% in group 3; and 5.4% in group 4. The total count of RBC and WBC were within normal range. 2. Bone marrow : No significant changes in bone marrow blood findings were observed. 3. Histologic findings : 1) Tonsil : Depletion of small lymphocytes was noted in the perifollicular area and periphery of the follicles in ALS treated groups. In one case in group 1, lymphoid proliferation and infiltration of reticulo-histiocytes were noted in perifollicular area. 2) Spleen : Depletion of small lymphocytes was noted in the perifollicular area, the periphery of the follicles and around the central arterioles in all ALS treated groups. In one case treated with ATS, reticulohistiocytic infiltration was noted around the central arteriole. All three cases of ATS with ASS treatment showed lymphoid hyperplasia in the perifollicular area and periphery of the follicles. No changes were observed in the red pulps. 3) Mesenteric lymph node : Depletion of small lymphocytes was noted in the outer cortex, corticomedullary junction and paracortical area in all ALS treated groups. The paracortical areas were partially replaced by reticulo-histiocytes in one case in both group 1 and 2, and in all cases in group 3. 4) Cervical lymph node : Depletion of small lymphocytes was noted in the outer cortex, corticomedullary junction and paracortical area in all ALS treated groups. Infiltration of reticulo-histiocytes was observed in the paracortical area in all ATS and ATS with ASS treated groups and in one case in the ASS treated group. 5) Inguinal lymph node : Depletion of small lymphocytes with partial replacement of reticulohistiocytes were noted in the outer cortex, cortico-medullary junction and paracortical area in all ALS treated groups. 6) Appendix : No significant histologic changes were noted in any groups. The germinal centers were intact in all lymphoid tissues observed. Reduction in the small lymphocyte population and creeping of some reticulo-histiocytes were noted in the medullary cords in one case each of mesenteric and cervical lymph nodes treated with ATS and two cases of inguinal lymph nodes treated with ATS with ASS.