Standard treatment regimens for haematological malignancies include myeloablative chemoradiotherapy and subsequent rescue by stem cell transplantation. However, these treatment regimens have significant associated mortality and morbidity, and disease recurrence remains a problem. One alternative approach is the targeted delivery of radiotherapy to the marrow using a bone-seeking agent labelled with an appropriate radioisotope, with the aim of delivering a potentially ablative radiation dose to marrow while minimising non-haematological toxicity. Pharmacokinetics and radiation dosimetry for a commercial preparation of samarium-153 ethylene diamine tetramethylene phosphonate (EDTMP; Quadramet) were evaluated in 43 tracer (average dose 740 MBq) studies of 42 patients with haematological malignancies. Measurements of 24-h retention were also available following infusion of 18-48 GBq in 15 patients. Quadramet cleared rapidly from the tissue, with a median biological half-life of 1.4 h. Activity taken up by the skeleton was firmly bound, with activity decreasing according to physical half-life at 24 h in 29 of the 43 cases. The percentage activity retained in the skeleton at 24 h with tracer doses was high (62%+/-13%), although this decreased to approximately 30% with therapy infusions. Because of this decrease in retention, the maximum feasible therapy activity for this formulation of Quadramet is 35 GBq. Median absorbed marrow radiation dose was 0.78 Gy/GBq in tracer studies: the decreased retention at high activities means that this corresponds to a median dose of 12 Gy for 35 GBq administered activity. It is possible to use 24-h retention as a rough guide to marrow dose in individual patients. In tracer studies, median bladder radiation dose was 0.22 Gy/GBq and radiation dose to the liver was very conservatively estimated at 0.2 Gy/GBq. After therapy infusions of up to 50 GBq in 37 patients, non-haematopoietic toxicity was not seen in any patient. In addition, myelosuppression was achieved without evidence of myelofibrosis. The residual dose rate to marrow fell to a level acceptable for stem cell re-infusion by 2 weeks after administration.