A calibration method is described for measuring absolute radioautographic sensitivities under various experimental conditions. Sensitivities to 3H and 35S radiation, i.e. ratio of developed grains to radioactive decays in the specimen, were determined with Ilford L4 and Kodak NTE emulsions. The highest values obtained in monolayers of emulsion were ⅛ for 3H and 1/21 for 35S. The influence of various experimental parameters on sensitivity is described, and the possibilities for quantitative electron microscope radioautography are discussed.