An experiment was conducted to reexamine earlier observations that lymphomas could develop from lymphocytes present in nonirradiated thymuses grafted into thymectomized, fractionally (170 R, 4 doses) irradiated mice by using B10.Thy 1 congenic donor-host combinations. The results indicated that: (a) 37 of 91 thymectomized, fractionally irradiated B10.Thy 1.2 mice which were grafted s.c. with 7-day-old thymuses from B10.Thy 1.1 donor mice had developed frank lymphomas between 90 and 270 days after thymus grafting; (b) 28 of 37 lymphomas developed in this group were typed individually with respect to Thy 1 alloantigens by the cytotoxicity assay using monoclonal anti-Thy 1.1 (T-11-D7) and anti-Thy 1.2 (F7D5) antibodies plus complement. It was shown that 21 thymic lymphomas (75%) had originated from lymphocytes of the nonirradiated thymus grafts and 5 tumors (17.9%) from cells of the irradiated hosts; 2 thymic lymphomas (7.1%) manifested no Thy 1 antigens; (c) lymphoma cells originated from both nonirradiated thymus grafts and irradiated hosts possessed chromosome abnormalities, which were mainly numerical changes of some chromosomes or polyploidizations.

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Supported by Special Project Grant for Carcinogenesis Study from the Science and Technology Agency and Grant-in-Aid 56010053 from the Ministry of Education, Science, and Culture, Japan.

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