IN recent years it has repeatedly been proved that neonatal or very young animals are very susceptible to carcinogenesis induced by different agents. One possible explanation is that tissues of new-born animals contain a large number of incompletely differentiated cells, particularly responsive to carcinogenic stimuli. It is relevant to this speculation that a close correlation has been noted between susceptibility of mice to develop malignant lymphomas and the presence in the thymus of a great concentration of immature cells, whether by reason of early age1, genetic constitution2, whole-body X-irradiation3, or regenerative changes in implanted thymus4. In thymectomized, virus-inoculated5 or X-irradiated4 mice, implantation of new-born thymus proved much more effective than adult thymus in restoring leukaemogenesis.