These experiments show that small lymphocytes from the thoracic duct of rats are normally a mixture of thymus-derived and marrow-derived cells, and define the traffic areas in lymphoid tissues through which the two populations recirculate.

Thoracic duct lymphocytes were labeled in vitro with uridine-3H and their histological distribution in the lymphoid tissues of normal recipients was demonstrated by radioautography. Labeled lymphocytes occupied two adjacent areas distinguished by a marked difference in the intensity of labeling; heavily labeled cells were found in thymus-dependent traffic areas of lymphocyte recirculation, while lightly labeled cells localized in the thymus-independent follicular areas around germinal centers.

A corresponding heterogeneity of uridine uptake among small lymphocytes from normal donors was demonstrated by sedimentation at 1 g; slowly sedimenting cells incorporated little uridine and localized in follicular areas after transfusion while rapidly sedimenting cells incorporated more uridine and localized in thymus-dependent areas after transfusion.

Experimentally prepared marrow-derived small lymphocytes behaved in sedimentation studies and after transfusion like a pure population of the lightly labeled small lymphocytes in normal lymph. Artificially reconstituted mixtures of marrow-derived and thymus-derived lymphocytes were qualitatively indistinguishable from normal lymphocyte populations.

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