In normal CBA/H mice implanted under the kidney capsule with eight CBA/H.T6T6 neonatal thymus lobes it was observed that the percentage of marked thymus-graft derived T cells in the periphery, after building up to a peak, showed a biphasic exponential decline. The initial decline was very rapid and appeared to be due to loss of the thymus-graft derived cells from the system. The later decline was slower and was the same as that of an introduced cohort of lymph-node lymphocytes. The second rate of decline was, however, considerably more rapid than that of lymph-node cohort in non thymus-grafted mice. We conclude that in multiply thymus-grafted mice the flow of cells through the T-cell pool is more rapid than in normal mice and that in this sense the thymus can be thought to drive the lymphoid system.