AbstractA method for the biosynthesis of15N‐labelled S‐adenosylmethionine from15NH4+and L‐methionine by yeast is reported. Culture conditions suitable for economy in the utilization of the isotope were developed, and the use of γ‐irradiated yeast (1 Mrad) for this purpose was explored. The irradiated yeast proved as effective as non‐irradiated yeast, but it did not offer any advantage.15N‐Adenine was obtained from S‐adenosylmethionine by hydrolysis; its isolation by ion exchange chromatograph