212 bi 223 ra 219 rn 215 po 211 bi 225 ac 221 fr 217 at 213 bi and 225 fm decay energies of useful alpha particle emitting radionuclides are preferably 2 000 9 000 kev more preferably 3 000 8 000 kev and most preferably 4 000 7 000 kev chelators such as those disclosed in u s pat no 5 753 206 especially thiosemi carbazonylglyoxylcysteine tscg cys and thiosemicarbazinyl acetylcysteine tsca cys chelators are advantageously used to bind soft acid cations of tc re bi and other transition metals lanthanides and actinides that are tightly bound to soft base ligands especially sulfur or phosphorus containing ligands it may be useful to link more than one type of chelator to a peptide e g a hard acid chelator like dtpa for in iii cations and a soft acid chelator like tscg cys for tc cations because antibodies to a di dtpa hapten are known barbet 39 395 supra and are readily coupled to a targeting antibody to form a bsab it is possible to use a peptide with a cold di dtpa chelator e g not chelated with a radioisotope and a chelator with a radioisotope in a pretargeting protocol for targeting the radioisotope to diseased tissue one example of such a peptide is ac lys dtpa tyr lys dtpa lys tscg cys nh 2 seq id no 2 this peptide can be preloaded with in iii and then labeled with 99m tc cations the in iii ions being preferentially chelated by the dtpa and the tc cations binding preferentially to the thiol containing tscg cys other hard acid chelators such as nota dota teta and the like can be substituted for the dtpa groups and mabs.