A method has been developed to produce very uniform monolayers of silver bromide crystals over ultrathin sections for electron microscopic autoradiography. Diluted nuclear emulsions are centrifuged to sediment its crystals over the sections. By changing the dilution and amount of the centrifuged emulsion, the density of the produced layer can be controlled at will. The position of the grains is secured by a collodion film. The method makes it possible to coat several grids at the same time with a very similar emulsion layer and is reproducible from experiment to experiment. Various factors that affect the grain distribution have been studied. The localization of I 125 in the thyroid has been studied to test the method.