Radiolabeled single-chain antibody fragments (scFvs) are potentially valuable reagents for imaging and therapeutics. However conventional techniques for binding them to radioisotopes are cumbersome and often reduce their biological activities. On page 897, Plückthun and colleagues describe a new method of formulating technetium-99m, so that it binds conveniently and efficiently to histidine-tagged scFvs. This new chemistry will also make it easier to attach technetium and rhenium to a host of other molecules for therapeutic and diagnostic applications.