When AKR mice are irradiated with a sub-lethal dose (4 times 175 R), thymic lymphosarcomas (L.S.) occur earlier than in controls. This accelerated leukaemogenesis is not inhibited by syngenic restoration with bone marrow cells (BM). Using the AKR/T1ALD substrain which bears 38 chromosomes with 1 metacentric markers, it has been shown that AKR radio-chimaeras restored by T1ALD BM developed two kinds of L.S. : (i) early (radiation-induced) L.S. originating mainly from host cells surviving irradiation and (ii) late L.S from donor cells. The present experiments were investigate the potential influence of normal allogenic thymic cells, with or without syngeneic B.M., on the incidence, latency and origin of LS appearing in irradiated AKR recipients. Adding C3H allogenic thymic cells to syngenic B.M. increases the percentage of early L.S. whose latencies are unchanged. Besides, when C3H thymic cells are injected to irradiated controls without syngenic B.M. cells, L.S. are seen to occurr significantly earlier than in just the irradiated animals alone. In radio-chimaeras restored by allogenic thymic cells and syngenic B.M., except in one case, all the L.S. were seen to originate from B.M. cells. The interpretation of these results depends on the possible role of allogenic thymic cells on host cells surviving the irradiation, or the exogeneous B.M. In the first case, allogenic thymocytes could induce a graft versus host reaction increasing the post-irradiation depletion of lymphoid system and hastening thymic endoregeneration which is supposed to be the first step towards leukaemogenesis. The second hypothesis, which seems the most likely, would be that C1H thymic cells could selectively act on host cells surviving irradiation and enhance the differentiation of haemopoietic precursors at the expense of the lymphoid cells. Alternatively the thymic cells may co-operate with exogenic B.M., and stimulate their differentiation thus triggering them out of a stage whereby they could become a selective target for the Gross virus.