SUMMARYLethal “homologous disease” has been produced in (CBA × T6)F1mice within 21 days by sublethal irradiation and injection of C57BL spleen and lymph node cells. Groups of injected recipients were sacrificed at regular intervals to provide material for serial histology and for quantitative estimation of donor and host-type dividing cells in the spleen (by T6 marker-chromosome technique). The results indicate that injected C57BL cells proliferate very intensively in the spleen of sublethally irradiated (CBA × T6)F1hosts, so that from day 5 after injection throughout the observation period (5–21 days) practically all dividing spleen cells are of C57BL origin. Parallel histological examinations of the spleen disclosed an initial, massive proliferation of blast cells followed by a quite sudden decrease in their number. This eventually resulted in a typical picture of a severe lymphoid atrophy in succumbing mice. The results are discussed with regard to the possible mechanism of lethal homologous disease in sublethally irradiated mice injected with homologous cells.