153Sm-EDTMP (ethylenediaminetetramethylene phosphonate), prepared from a kit, was administered to 28 patients in a clinical trial of therapy for painful skeletal metastases unresponsive to all conventional treatment. The 103 keV gamma emission of 153Sm was utilized for prospective individual estimation of beta radiation absorbed dose to red marrow to minimize myelotoxicity and provide optimum internal radiotherapy to skeletal metastases in each patient. Pain relief occurred within 14 days of administration of 153Sm-EDTMP in 15 of 19 patients (79%) who could be evaluated at 6 weeks, when clinical response was maximal. Duration of response ranged from 4 to 35 weeks. Recurrence of pain responded to retreatment with 153Sm-EDTMP in five of eight cases. No dose-response relationship was apparent for pain relief but reversible myelotoxicity was frequently observed at radiation absorbed doses to bone marrow greater than or equal to 270 cGy. Dosimetry calculation was based on pharmacokinetic studies of a tracer administration of 153Sm-EDTMP in each patient. Assumptions inherent in this prospective method of predicting dose to bone marrow were validated experimentally. Biodistribution studies in rats demonstrated rapid skeletal uptake and long term retention of 153Sm-EDTMP in bone over 5 days. Urinary clearance accounted for 40% of injected dose, and less than 1.0% of administered activity was retained in non osseous tissue. Microdensitometry of autoradiographs of sheep vertebra and femur confirmed surface uptake of 153Sm-EDTMP in cortical bone and demonstrated relatively high trabecular bone activity which is the major component of radiation absorbed dose to bone marrow. Haematological studies in rabbits showed 153Sm-EDTMP-induced myelotoxicity to be transient and no histopathological abnormalities were demonstrable with doses ten times greater than those administered to patients.