Adenine-8-C14 was used as a nucleic acid precursor to provide a permanent label from the time of the cell''s formation. The course of appearance and disappearance of labeled cells yielded the following estimates: mature neutrophils spend less than a day in marrow and somewhat more than a day in the periphery. Lymphocytes fall into 2 classes[long dash]medium and large cells, which are rapidly renewed, and small cells, which live at least a week.