By means of micro-autoradiography incorporation studies of labelled compounds can be performed at the cellular level. The procedure described here aims at determining the amount of substances in terms of moles, incorporated into single cells. 14C is used as a label, since in contrast to tritium, the influence of β-self-absorption of this isotope is negligible. Every autoradiograph is exposed together with a standard consisting of 14C-methacrylate. Hereby the relation of grain density to radioactivity in an autoradiograph is given. Grain counts are performed in an incident light microphotometer which makes use of the linear relationship between silver grain number and amount of light reflected by the grains. Therefore, measurements are performed in an integrating manner. The usual transmitted light is available for selecting the labelled cells for measurement.