Prolonged survival and restoration of immunologic functions could be induced in neonatally thymectomized C3Hf/Bi mice by i.p. grafting at 30 days of age of two thymuses from syngeneic or hemiallogeneic donors. Allogeneic thymus grafts were less effective and early mortality was high, suggesting participation of a graft-vs.-host reaction.

Tolerance to skin of the same origin as the thymus graft was observed in some hemiallogeneic combinations but not in others.

Discriminant spleen assays of the restored animals always showed an immunocompetent host component, and also indicated specific tolerance when tolerance to skin was present.

Chromosome analysis of neonatally thymectomized mice treated with (C3H × T6)F1 thymuses indicated low numbers of donor mitoses in the thymus graft and in superficial lymph nodes, especially when tolerance to (C3H × T6)F1 skin was present.

Attempts to transfer tolerance induced by thymus grafts with spleen cells injected into C3Hf/Bi newborns failed, regardless of the duration of tolerance in the donor.

Tolerance induced by injecting spleen cells into either normal C3Hf/Bi newborns or neonatally thymectomized 5-day-old C3Hf/Bi mice could be easily transferred by injecting 5 × 106 spleen cells from tolerant donors into newborn syngeneic mice.

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