The popliteal nodes of sheep were perfused continuously for over 100 hours with a solution containing 3H-thymidine. The labelling pattern of the lymphocytes in the efferent lymph from the perfused nodes showed that under normal conditions not more than 4 per cent of these lymphocytes were actually produced in the node. The transitional cells, large lymphocytes and cells of the plasma cell series which appeared in the lymph following antigenic stimulation of the node were all produced in the node.

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