Auger electron therapy exploits the cytotoxicity of low-energy electrons emitted during radioactive decay that travel very short distances (typically <1 μm). 201Tl, with a half-life of 73 h, emits ∼37 Auger and other secondary electrons per decay and can be tracked in vivo as its gamma emissions enable SPECT imaging. Despite the useful nuclear properties of 201Tl, satisfactory bifunctional chelators to incorporate it into bioconjugates for molecular targeting have not been developed. H 4pypa, H 5decapa, H 4neunpa-NH 2, and H 4noneunpa are multidentate N- A nd O-donor chelators that have previously been shown to have high affinity for 111In, 177Lu, and 89Zr. Herein, we report the synthesis and serum stability of [ nat/201Tl]Tl 3+complexes with H 4pypa, H 5decapa, H 4neunpa-NH 2, and H 4noneunpa. All ligands quickly and efficiently formed complexes with [ 201Tl]Tl 3+that gave simple single-peak radiochromatograms and showed greatly improved serum stability compared to DOTA and DTPA. [ natTl]Tl-pypa was further characterized using nuclear magnetic resonance spectroscopy (NMR), mass spectroscopy (MS), and X-ray crystallography, showing evidence of the proton-dependent presence of a nine-coordinate complex and an eight-coordinate complex with a pendant carboxylic acid group. A prostate-specific membrane antigen (PSMA)-targeting bioconjugate of H 4pypa was synthesized and radiolabeled. The uptake of [ 201Tl]Tl-pypa-PSMA in DU145 PSMA-positive and PSMA-negative prostate cancer cells was evaluated in vitro and showed evidence of bioreductive release of 201Tl and cellular uptake characteristic of unchelated [ 201Tl]TlCl. SPECT/CT imaging was used to probe the in vivo biodistribution and stability of [ 201Tl]Tl-pypa-PSMA. In healthy animals, [ 201Tl]Tl-pypa-PSMA did not show the myocardial uptake that is characteristic of unchelated 201Tl. In mice bearing DU145 PSMA-positive and PSMA-negative prostate cancer xenografts, the uptake of [ 201Tl]Tl-pypa-PSMA in DU145 PSMA-positive tumors was higher than that in DU145 PSMA-negative tumors but insufficient for useful tumor targeting. We conclude that H 4pypa and related ligands represent an advance compared to conventional radiometal chelators such as DOTA and DTPA for Tl 3+chelation but do not resist dissociation for long periods in the biological environment due to vulnerability to reduction of Tl 3+and subsequent release of Tl +. However, this is the first report describing the incorporation of [ 201Tl]Tl 3+into a chelator-peptide bioconjugate and represents a significant advance in the field of 201Tl-based radiopharmaceuticals. The design of the next generation of chelators must include features to mitigate this susceptibility to bioreduction, which does not arise for other trivalent heavy radiometals.