Thymocytes from C57BL mice immunized with the DBA/2 lymphoma L5178Y exert in vitro an immunologically specific cytotoxic action against the target cells in the presence of bone-marrow cells. Neither the nonimmune bone marrow nor the immune thymocytes are by themselves cytotoxic. The cells in the bone marrow which take part in the cytotoxic action adhere to glass and are sensitive to anti-macrophage serum. These bone-marrow cells can also be rendered specifically cytotoxic by exposure to the supernatant obtained from a culture of immune thymocytes with the specific target cells. The thymocytes before they are confronted with the specific target cells are very radiosensitive; however, on coming into contact with the target cells, an immunologically specific increase in RNA synthesis occurs and thereafter the thymocytes' capacity to render bone-marrow cells cytotoxic is relatively radioresistant.

Two classes of immune lymphocytes occur in mice immunized with allogeneic cells, those that are capable of killing target cells directly and those that produce a factor capable of rendering macrophages (or monocytes) specifically cytotoxic. In the thymus of immune animals only the latter are found while both categories are present in the spleen and lymph nodes of immune animals.

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