Sensitivity in electron microscope autoradiography was determined for 125I. Values are given using Ilford L4 and Kodak NTE emulsions combined with different developers. The extent of self-absorption as a function of section thickness and heavy metal staining and the effect of radiation dose (dose dependence) were assessed. It was found that the over-all efficiency for 125I was better than that for tritium and that, as is the case with tritium, there is a distinct "dose dependence" especially when Microdol X is the developer. Self-absorption studies indicate that self-absorption is increased by about 15% when the specimen thickness is increased from 300 to 1000 Å. An increase of under 15% is also introduced by heavy metal staining of sections in this thickness range.