Transplantation of homologous bone marrow of mice following supralethal doses of whole-body irradiation was followed by a high incidence of death due to “late disease” presumably resulting from the production of antibodies against the host by immunologically competent elements of the transplanted bone marrow. The incidence of secondary disease was somewhat lower if the bone marrow donors were pretreated with cortisone, nitrogen mustard, or both. Such pretreatment decreased the number of lymphocytes in the bone marrow and reduced the levels of circulating γ-globulin. Treatment of the recipients with cortisone subsequent to bone marrow transplantation had little effect.