THE observations recorded here were made in the course of a study, of which one part is described elsewhere [Barry et at. 1928] of the action of arsenic upon oxidation-reduction systems. We endeavoured at first to examine the fixation of arsenic by glutathione which has been described in several papers by Voegtlin and his fellow-workers [1923, 1925]. To this reaction they have assigned great importance in the detoxication of arsenical compounds, and the process should be of considerable interest in connection with arsenic cancer. Our experiments were made with the skin and muscle of the rat, the Jensen sarcoma, the liver of the calf and fowl, and yeast. We could not find the expected constant difference in the strength of the nitroprusside reaction between extracts of these materials which had, or had not, been treated with solutions of arsenites. Evidently the effect upon the nitroprusside reaction of the combination of arsenic with glutathione, which Voegtlin seems to regard as beyond doubt2, requires investigation. However, in the course of these experiments we were led to note the liberation by a number of physical and chemical agents of substances giving the nitroprusside reaction, and these observations are recorded here. TECHNIQUE. The experiments, begun with mammalian cells, have been carried on up till now with yeast, a very convenient material. 2 g. compressed yeast were suspended in 20 cc. of fluid, which was either Pasteur's solution, 10 % sucrose or dextrose in water, or distilled water alone. We have not checked every result given below with yeast suspended in each one of these four media, but in general the effects appear to be of the same relative order whichever medium is used. As a rule the order of increasing strength of nitroprusside ' This temperature is about 100 above the optimum for the growth of yeast, but this does not affect the comparative value of the results, and the investigation brought forward here was concerned with a search for cytol,ytic agents rather than with the growth of yeast.