1. The regeneration of cortical tissue in isologous thymic grafts in thymectomized C57BL mice has been followed by semi-serial histological sections at frequent intervals after implantation. Cortical regeneration is nearly complete by the 14th day in nonirradiated hosts and reaches its maximum in irradiated hosts at about the same time. The incidence of cortical regeneration in the grafts in irradiated hosts is sufficient to account for the observed incidence of lymphomas at later intervals.

  2. Prior systemic irradiation of the recipients impairs the incidence and extent of cortical regeneration in the grafts. Thigh-shielding during such irradiation permits the grafts to regenerate as completely as in nonirradiated hosts.

  3. The age of the thymus donor has an important effect on the incidence of tumors in the grafts, the peak susceptibility occurring in the implants from the youngest donors.

  4. The sex of the donor prior to puberty has no influence on the susceptibility of the implant to lymphoma development.

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These investigations were supported by grants-in-aid from the National Cancer Institute, National Institutes of Health, U.S. Public Health Service, the Jane Coffin Childs Memorial Fund for Medical Research, and an American Cancer Society Institutional Cancer Research Grant to Stanford University.

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