SINCE fluoride has a strongly inhibiting effect on many enzymes, its unsuspected presence, even in small amount, may cause serious disturbance in biochemical work. It may be well, therefore, to direct the attention of biochemists to the fact that some samples of sodium phosphate contain fiuoride. Recently in this laboratory it was found that a specimen of sodium phosphate recryst, purchased in the ordinary course as a reagent, strongly inhibited alcoholic fermentation by dried yeast. On examination it was found that fluoride was present in sufficient quantity to be easily detected qualitatively. Judging by its biochemical effect, about 0.1–0.5 per cent of fluoride (as sodium fiuoride) must have been present in the sodium phosphate, but no quantitative chemical estimation was made. A single crystallisa tion of 25 gm. of the salt from 100 ml. of water gave a, product which reacted with yeast and sugar in the normal manner.