A TWO EMULSION AUTORADIOGRAPHIC TECHNIQUE AND THE DISCRIMINATION OF THE THREE DIFFERENT TYPES OF LABELLING AFTER DOUBLE LABELLING WITH 3H‐ AND WC‐THYMIDINE

The first part of the paper deals with a two emulsion autoradiographic technique for double labelling experiments with 3H- and 14C-thymidine which permits a clear discrimination of the different types of labelling. In the second part the application of this technique to cell kinetic studies is discussed. Accurate discrimination between the different types of labelling, namely purely 3H-, purely 14C- and double (3H + 14C) labelling, is only possible if the activity ratio of 3H- to 14C-thymidine is sufficiently high. This condition is necessary for a reliable distinction between those grains in the first emulsion which are due to true 3H-labelling and spurious grains which are simultaneously produced in the same emulsion by 14C-beta-particles. Experiments are described to determine the required activity ratio of 3H- to 14C-thymidine.