Iodine-125 as a protein label in immunology.

Iodine-125 has the relatively long half-life of 60 days, and emits 27.3- to 35.5-kev photon radiation, permitting counting in an ordinary well crystal scintillation counter with 30 to 40 percent efficiency. The emission of low-energy electrons having the same general energy range as tritium beta radiation electrons allows high-resolution autoradiography. These properties, together with the other advantages of radioactive iodine, make this nuclide a particularly satisfactory label for immunologically active proteins.