Adoptive transfer experiments were performed to define the immunological role of thymus and bone marrow cells in the induction of delayed hypersensitivity (DH). The results indicated the following, (a) Bone marrow from immune donors contained cells capable of being stimulated by antigen to initiate the expression of DH. (b) Bone marrow from nonimmune or tolerant donors contained cells that were needed to complete the expression of DH after the infusion of immune lymph node cells. (c) Normal bone marrow and thymus cells cooperated in the irradiated recipient to induce the most vigorous skin reactions to specific antigen; these reactions were seen only when the recipients were stimulated by antigen. Either cell type alone was ineffective. (d) In the presence of tolerant bone marrow cells, thymus cells from immune donors gave a more vigorous response than did thymus cells from normal or tolerant donors. (e) There was suggestive evidence that thymus cells were the source of trigger elements that initiated DH. (f) Antigen in the irradiated recipient was necessary to induce DH after infusion of bone marrow cells alone, or bone marrow and thymus cells together.

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