Effect of the irradiation of the thymus on the development of l' cells was investigated. C57BL/6(H-2b, Thy-I. 2) mice were irradiated without shielding or with shielding over the thymus or over the right leg and tail, and they were transferred with bone marrow cells from BI0. Thy-I. 1 (H-2b, Thy-I. 1) mice. After the reconstitution, the proportion of donor-type Thy-I. 1 bearing cells were analysed by flow cytofluorometry. In the whole-body irradiated mice thymus cells were nearly completely

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... aced by donor derived cells within 21 days. In thymus-shielded mice and right-Ieg-and-tail-shielded mice, about half of the thymus cells were donor-type cells at 28 days after the reconstitution. The development of donor-derived cytotoxic l' cell precursors (CTLp) and helper l' cells (Th) in the thymus of these recipient mice was investigated. In order to increase the sensitivity of detection, concanavalin A-mediated polyclonal assay system was used for both CTLp and Th, and the PN A-cell fraction of thymocytes was used as a source of functional cells. In wholebody irradiated mice and in right-Ieg-and-tail-shielded and irradiated mice, CTLp developed. before Th did, whereas the reverse was true in thymus-shielded and irradiated mice. These results strongly suggested that the capacity of the thymus to allow the proliferation of donor type Thy-l bearing cells and the development of CTLp in the thymus was independent of whether or not this organ was irradiated. On the other hand, the development of functional Th was obviously retarded by the irradiation on the thymus, suggesting that the radiosensitive thymic element plays a critical role for the generation of Th.