Abstract: Amines, including proteins and polypeptides covalently coupled with a chelating agent such as diethylenetriaminepentaacetic acid (DTPA) can be labeled with .sup.99m Tc in the presence of a stannous reducing agent at near neutral pH by maintaining the protein concentration at a low level and the DTPA concentration at a high level. The .sup.99m Tc is labeled to the protein through the DTPA sites linked to the protein. Utilizing .sup.111 In labeled proteins as controls, it is shown that the .sup.99m Tc labeled proteins are labeled at the DTPA sites and that they exhibit in vivo stabilities equal to that of the .sup.111 In labeled proteins.