Peripheral blood lymphocyte levels attained in adult rats that had been thymectomized at birth were markedly reduced if the two or three immediately preceding generations of rats from which they were bred had been thymectomized as neonates. The capacity of thymectomized animals to survive to adult life was also drastically reduced if they were bred from thymectomized stock. Pregnancy as a result of mating with syngeneic or allogeneic males produced an ephemeral increase in peripheral blood lymphocyte levels of third and fourth generation thymectomized females. These observations are most readily explicable on the basis of a thymus-derived humoral influence acting directly or indirectly to influence the circulating lymphocyte population.

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