Thirty-two adult monkeys were given a single dose of total-body x-irradiation of 650, 700, or 800 r. An attempt was made in 24 of the animals to alter the acute phase of the radiation injury to hematopoietic tissues sulfficiently to prolong the life of the animals by post-irradiation administration of homologous bone marrow. The other 8 animals served as irradiated non-treated controls. Of the 24 bone marrow treated monkeys, 5 survived for more than 30 days post-irradiation. However, a total of 16 animals showed some recovery of the peripheral blood elements toward normal, which seems to indicate that the bone marrow graft may have been functional in these animals. The causes of death in the face of a recovering bone marrow are discussed with regard to immunological reactions, including the "foreign bone marrow reaction," and other contributing factors.