Abstract: A series of stable complexes of ~(153)Sm has been produced using multidentate acetate and phosphonate ligands. Biodistribution Studies in rats showed varying degrees of bone and soft-tissue uptake for these complexes. Of the complexes studied ~(153)Sm-EDTMP showed the best combination of high bone uptake, low nonosseous uptake, and rapid blood clearance in rats and rabbits. Lesion/normal bone ratios were determined from digitized images obtained using a drill hole model in rabbits and found to be 17: 1. Dog studies confirmed the ~(153)Sm-EDTMP results obtained in rats and rabbits. Dogs with spontaneous bone tumors were used to evaluate the effiealy of ~(153)Sm-EDTMP for its potential use in treatment of metastatic bone neoplasia in man. ~(153)Sm-EDTMP produced some degree of palliation in 80％ of dogs. The only clinical abnormalities noted were a reduction in platelets and white blood cells 2 to 3 weeks post injection with return to normal by 6 weeks. Based on these excellent biodistribution charaeteristies and results. ~(153)Sm-EDTMP has been used as a therapeutic agent to treat bone cancer.