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Extracorporeal measurement of incorporated radioactive organic compounds needs labelling with gamma emitters. The elements of organic compounds have only very short-lived gamma-emitting isotopes. (Organic compounds, therefore, up to now were labelled by more or less stable chemical coupling with gamma-emitting isotopes of the halogens or of certain metals as, for instance, chromium-51). The use of the C-11 isotope with a half-life period of about 20 min, emitting gamma radiation due to the annihilation radiation of its positrons, is very problematical, because these very short-lived nuclides can be used only at the place of their production by nuclear engineering. But especially these nuclides are very interesting just because of their short half-life period, the radiation load being lessened thereby and, in addition, the organic compound remaining chemically unchanged. Utilizing the (gamma, n) reaction, we have succeeded in labelling organic molecules in solid state directly within the electron beam due to the retarding of particles from a linear electron accelerator, and the absolute as well as the specific yield was satisfying. Till now, we have tested almost exceptionally amino-acids; during experimentation with rats, an example of the practicable utilization in nuclear medicine was set by means of the amino-acid methionine. This was confirmed by results of comparative studies using C-14 and Se-75 labelled methionine (Se-methionine respectively).