When neonatally thymectomized CBA mice were implanted at 9 to 12 days of age with Millipore diffusion chambers (pore size, 0.1 µ) containing either syngeneic or allogeneic neonatal thymus, they were subsequently found to have the capacity to reject skin homografts and to form antibodies to sheep erythrocytes. In spite of displaying restored immune reactivity, thymectomized mice bearing thymus-filled diffusion chambers still had a lymphopenia and diminished numbers of small lymphocytes in their spleens, lymph nodes and Peyer's patches. Comparison of the lymphoid organs of these mice with those of the thymectomized control mice did not reveal any appreciable difference in the numbers of primary follicles or small lymphocytes. It is postulated that the thymus humoral factor induced immunological competence in lymphoid cells which had left the thymus prior to neonatal thymectomy. The paucity of circulating and tissue small lymphocytes in thymectomized animals, the immune reactivity of which was restored by thymus tissue in diffusion chambers, argues against the theory that the thymus humoral factor has a lymphocytosis-stimulating effect.

There was no restoration of immune reactivity in those neonatally thymectomized mice which had been implanted with diffusion chambers containing neonatal or adult spleens, or adult lymph nodes. Thus, the competence-inducing factor is elaborated by the thymus but not by the spleen or lymph nodes.

Allogeneic (C57Bl) neonatal thymus tissue, enclosed within diffusion chambers, had the capacity to restore the immune reactivity of totally thymectomized CBA mice, not only to skin homografts of a totally unrelated strain (Ak), but also to grafts isogeneic with the donor of the allogeneic thymus. Therefore, there is no strain barrier to the action of thymus humoral factor.

To explain the apparent lack of full participation of thymus lymphocytes in immune reactions it is postulated that thymus lymphocytes are functionally immature in situ, and that they leave the thymus before attaining immunological competence. In the periphery, they undergo further maturation under the influence of the competence-inducing factor produced by the thymus.

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