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Following injection of antithymocytic serum (ATS) in Balb/C mice, increased decay of lymphocytes in lymphatic nodes occurs during the first 6 days, estimated by dye exclusion tests on unfixed cell suspensions. Decay in the thymus remains at normal values. Organ weights of the thymolymphatic system remains at control levels during the first weeks after injection. Initially the number of blood lymphocytes drops while the output from the thoracic duct remains stable. During the first hours after ATS-injection, the number of lymphocytes in postcapillary venules are reduced. During the first days, the lymph nodes show accumulation of lymphocytes in the cortical nodules along with depletion of the paracortical area. The thymus stayed morphologically normal, with the exception of a light depletion of the marrow during the first day after injection. Results are discussed in the light of recently reported investigations, and it is emphazied that the increased decay of lymphocytes in the nodes may be interpreted as an expression of a shift in population from longlived, small lymphocytes of thymus origin to short-lived small lymphocytes of bone marroworigin.