Strontium-89 is preferentially taken up at sites of increased bone mineral turnover, its uptake adjacent to malignant metastases being up to five times greater than for normal bone. Strontium-89 is also selectively retained in bone adjacent to metastatic sites. The initial therapeutic ratio is therefore good and increases with time. The pharmacokinetics of strontium-89 thus favor the objective of achieving a clinically effective beta radiation dose to tumor deposits while minimizing radiation exposure to healthy tissue. Clinical studies show that 150 MBq [corrected] strontium-89 can relieve bone pain in up to 80% of patients with prostatic metastases. Greater activities do not appear to increase this level of response. Once a response has been achieved, strontium-89 therapy can be repeated when pain recurs. Although hematologic toxicity at this dosage is low, strontium-89 should not be given to patients with evidence of significant bone marrow depression. With this precaution, no serious toxicity has been encountered when the agent is administered at the recommended doses.