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SUMMARYHydrogen-tritium exchange labelling methods proved to be unsatisfactory for the preparation of tritiated DL-a-tocopherol at high specific activity. The preparation of a-tocopherol specijcally labelled in the 5-methyl hydrogen positions is described : the method permits specific activities in the curies per millimole range to be readily achieved. The synthesis is also suitable jor preparing tocopherol doubly labelled with tritium and carbon-14. The stability of the tritiated vitamin on storage is discussed.The existence of the antisterility fat soluble vitamin, a,-tocopherol (vitamin E), was first recognised more than forty years ago (l, z). It was isolated in 1936 ( 3 ) , its structure (1) elucidated in 1937 (4) and first synthesised by Karrer et al. (6) in 1938. The naturally occurring ' tocopherols ' are predominantly a mixture of three compounds differing only in the number and position of methyl groups. Separation of these similar compounds is readily achieved by thin layer chromatography (6-8), a technique which is made use of in the purification of the labelled tocopherol. R, R2R3