A one vial method for labeling a protein, such as an antibody or antibody fragment, with a radiometal such as Tc-99m or a rhenium isotope, is disclosed. The method comprises contacting in a single vial a mixture comprised of a reducing agent and a protein molecule covalently bound to a sulfhydryl containing bifunctional coupling agent with Tc or Re in an oxidized state. A one vial kit for labeling a protein with Tc or Re is also disclosed.