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It is a well-known fact that thymic tissue enclosed in millipore diffusion chambers is capable of restoring both lymphopoiesis and immunological reactivity. Attempts to restore leukemogenesis by the same technique have so far proved unsuccessful. It seemed worthwhile to use the thymic tissue culture technique in order to obtain a quantitatively greater number of epithelial reticular cells. These cells were transplanted intramuscularly in the thigh of 40–50 day old W/Fu rats, thymectomized at birth and injected with Gross virus at 3–4 days of age. In 5 out of 11 animals a lymphoma appeared 6–8 months later at the site of implantation of thymic cells. In control animals (not injected with virus), islands of more or less morphologically normal thymus developed from the grafted cells. The findings suggest that, whereas reticular cells can restore lymphopoiesis and immunological reactivity, the presence of a whole thymus is essential for leukemogenesis to occur.