THE Marmite Food Extract Co., Ltd., writes, with reference to Mr. A. II. Keast's comments on the vitamin Bx potency of its yeast extract (NATURE, Nov. 3, p. 696), that the first estimation which showed a content of 840 international units per oz. has been confirmed in a later test on a mixture of eight different samples. The tests were carried out on pigeons by the method described by Coward, Burn, Ling and Morgan (Biochem. J., 27, p. 1719; 1933). Their letter has been referred to Mr. Keast, who points out that Marmite deteriorates with age, and that the pigeon cure method does not always give the same result as the rat growth method of assaying vitamin Bx in terms of the international standard. Coward et aL (loc. cit.) found, for samples of dried yeast, that the pigeon method (cure of head retraction in birds given a diet of polished rice) gave a higher value for the vitamin Bx potency than the rat method (growth of rats on a diet deficient solely in vitamin Bx): the same results were, however, obtained by both methods in the case of a soft yeast extract. They also point out that although the probable error of the pigeon test is much greater than that of the rat test, yet the former has the great advantage of being specific for the factor it is used to estimate.