BAKER'S yeast in the resting state normally contains a relatively large amount of potassium. In addition, it has been shown by a number of workers that during active metabolism (aerobic or anaerobic utilization of glucose1 or aerobic respiration of certain other substrates2) considerable net absorption of potassium can occur even though the concentration of potassium in the external medium may be very much less than that inside the cells. Accumulation of potassium, and of other cations, by yeast involves active transport of the ions across the cell membrane3, the inward movement of ions being mediated by relatively specific carrier mechanisms4 therein. Many facts require this interpretation and have been stated in detail elsewhere.