[en] This research work related to the radioactive labelling with tritium of the enzyme neuraminidase from Vibrio cholerae by an easily handled method. The reactive compound N-propionyloxysuccinimide, the ester of propionic acid and N-hydroxysuccinimide, offered a suitable labelling reagent. For comparison purposes an already known method of labelling neuraminidase with tritium by the oxidation of hydroxyl groups of the hydrocarbon chain of the enzymal protein and subsequent reduction of the aldehyde groups formed with tritiated sodium borhydride, was also carried out. The advantages and disadvantages of both methods are described in detail, in particular with regard to yields of radioactivity and the influence on enzyme activity. The fact that only 1 mg enzymal protein was available for each modification of the enzyme molecule posed particular problems and, as a consequence, extensive preliminary experiments had to be carried with another protein (beef serum album) in the same concentration range. (orig./MG)