Bone marrow is most often the dose-limiting organ in radiation immunotherapy. Controversy exists over optimal methods of estimating radiation dose to bone marrow. The authors compared findings in serial blood samples to findings in bone marrow biopsy samples as measures of bone marrow activity from which to calculate bone marrow dose. Peripheral blood samples and bone marrow biopsy samples were obtained from 11 female patients at 48 and 168 hours after infusion of iodine-131-labeled Mc5 antibody. Bone marrow biopsy data demonstrated markedly decreased specific activity compared with that measured in the peripheral blood. Activities at 48 hours after infusion ranged from 3% to 22% of the peripheral blood activity. These results indicate that activity concentration in the bone marrow is not equivalent to activity concentration in the blood for the Mc5 antibody. These results imply that a dose three to four times that indicated from serial blood samples could be tolerated by patients, provided the antibody-radioisotope does not bind to the marrow.