Some essential physical properties of the transmutation of iodine-128 are discussed and the range of its electrons calculated. Its suitability for autoradiographs is compared with that of tritium. The dose distribution around a point source and the dose absorbed by a cell nucleus labelled with and 3H are calculated. It is suggested that the biological toxicity of iodine-125 is not only due t o energy deposition of its radiation but also to molecular fragmentation caused by charge transfer in consequence of the Auger effect.