The development of techniques for observing tissues by electron microscopy has opened a whole new world of structure to the biologist. As the descriptive detail of cell organization has built up, the pressure for methods that would link these new structures to the biochemistry of the cell has also increased. It was inevitable that an attempt to apply the radio-isotopic tracer experiment at the electron microscope level should be made: it was equally certain that this attempt would involve autoradiography, since the nuclear emulsion is the only detector for radio-isotopes with powers of resolution anywhere near sufficient for the size of organelle under study. In attempting to marry the two approaches of electron microscopy and autoradiography, the logical starting-point has been to take the established product of the one technique—the conventional thin section of tissue—and attempt to adapt the methods and materials of autoradiography to it. To this day, the majority of publications in this field deal with material fixed in aldehydes, postfixed in osmium tetroxide, embedded in epon or araldite, and sectioned at 50 to 100 nm. Most of the information available deals with this type of specimen as the source of radioactivity.