THE investigations carried out during the past few years by Bourdillon and his colleagues at the National Institute for Medical Research on the formation of vitamin D have now led to the preparation of a pure crystalline compound, as recently reported by Askew, Brace. Callow, Philpot, and Webster, in NATURE for Oct. 31, p. 758. This compound, called by them ‘Calciferol’, is prepared by the irradiation of ergo-sterol in ethereal solution under strict anaerobic conditions, excess ergosterol being afterwards removed by fractional crystallisation and with the use of digitonin. The calciferol is separated from other products of irradiation by fractional crystallisation of the dinitro-benzoates and subsequent hydrolysis of the ester. The process has now been repeated qn a larger scale in the technical laboratories of Messrs. The British Drug Houses, Ltd., London. N.I. The sample of calciferol submitted is a white crystalline solid: it is stated to melt at 114.5 ° C., to have the same empirical formula as ergosterol, and to have the high vitamin D potency of 40,000 international antirachitic units per milligram. It is available for the use of research workers in tubes containing 100 mgm. The commercial production of pure vitamin D is a satisfactory culmination of the brilliantly directed but laborious research work of the Medical Research Council's investigators, and should lead to greater precision in therapeutics and to the possible discovery of new applications for its use.