Abstract: A modified method for the biosynthesis of L-cyctine-~(35)S from baker's yeast(Saccharomyces Cerevisiae) is described. The yeast was cultivated in a sulphur-depleted medium containing carrier-free Na_2 ~(35)SO_4 under bubbling of air during 48hours, yeast proteins were hydrolysed enzymatically into aminoacids, and L-cystine-~(35)S was isolated by ion exchange chromatography on a column. Radioactive yieldfor L-cystine-~(35)S is 16-19% and the obtained products have specific activities of 15-22 mCi/mmol and radiochemical purity better than 95%.