For the biosynthetic preparation of14C-erucic acid (C21H41COOH) by means of rape plants cv. sollux the plants were supplied with14CO2and additionally fed with14C-Sodium acetate after anthesis. After saponification of the extracted lipids the erucic acid was isolated and purified. The substance was identified by gas chromatography. The incorporation of the applied radioactivity (34 MBq14CO2; 37 MBq14C-natrium acetate) into the fatty acids amounted to 1,2 per cent. The eruric acid could be isolated from the fatty acids mixture with a specific radioactivity of 1,001 MBq/mmol and a purity of 97,2 per cent.