A marrow transfer technique was used to study the functional state of the marrow of lethally x-irradiated mice after intravenous injection with isologous or heterologous (rat) marrow. In the former case, it was found that the marrow behaved as functionally active mouse marrow (in terms of radiation protection) at 19 days postradiation; while it was essentially inactive at 14 days although it appeared to be regenerated, as judged histologically. Irradiated mice (800 r) injected intravenously with rat marrow exhibited ‘regenerating’ marrow, but it was not active in preventing death when injected into other irradiated mice. Rat bone marrow, administered intraperitoneally to lethally x-irradiated mice failed to elicit any 15-day survivors, even with 350 mg or 700 mg doses, per mouse. This latter dose is 70 times greater than the 10 mg of isologous mouse marrow, required to evoke 50% survival.