The preceding studies have established the following points: Intrathymic labeling of thymic lymphocytes provides an adequate marker system to detect the migration of thymus cells to peripheral lymphoid sites. In the newborn, this comprises a major portion of the total lymphocyte population in lymph nodes and spleen. In the adult, this migration is limited to specific portions of lymph nodes and spleen, i.e., those portions which serve the recirculating pool of small lymphocytes. Kinetic studies of labeling within the adult thymus indicate that large cells give rise to medium and small cells, which then migrate to the specific sites noted above. In the newborn, the kinetic pattern is similar to that of adults, with the single distinction that large cells also migrate, accelerating the tempo of migration in these hosts. The long-term fate and function of thymus cell migrants has not yet been determined.