SummaryThe neonatal injection of 7,12-dimethylbenz[α]anthracene (DMBA) into the low leukemic strain of CFW mice resulted in a 100% incidence of thymic lymphomas. Injection of normal, syngeneic bone marrow cells 1–21 days post DMBA treatment markedly reduced the tumor incidence but did not affect the mean latent period. At 30 days post bone marrow injection there was evidence of extensive re-population (approximately 80%) of both thymus and bone marrow in DMBA-treated animals given normal bone marrow 5–21 days after DMBA. When normal bone marrow was given 1 or 35 days after DMBA treatment, the number of donor-derived cells at 30 days post bone marrow injection was less extensive (approximately 50%) in both bone marrow and thymus. Therefore, the establishment of a successful chimera depended on the interval between DMBA treatment and bone marrow injection. At tumor development (90 or more days post DMBA treatment) the cells of both thymus and bone marrow of mice given bone marrow 5–35 days post DMBA treatment (87–176 days post bone marrow injection) but showing no visible signs of thymic lymphoma were still predominantly of donor origin (approximately 90%). However, at 100–150 days post bone marrow injection, no donor cells were found in the thymus and bone marrow of mice given normal bone marrow 1 day post DMBA injection. Direct cytologic examination of the induced thymic lymphomas showed that all were derived from host cells, even though in the same mice (provided the tumor had not disseminated) the bone marrow was still donor in origin. The exception to this occurred in mice given normal bone marrow 1 day post DMBA treatment. In these mice the dividing cells in the bone marrow of tumor-bearing mice were all host in origin, even though the tumors were confined to the thymus. Since the injected bone marrow resulted in a 50% reduction in the tumor incidence, it would appear that the ability of normal bone marrow to reduce the incidence of thymic lymphomas is not related to its ability to establish a permanent chimera.