A relationship between the intracellular concentration ratios [K+]/[Na+] and [Mg++]/[Ca++] has been found in a variety of cells and organelles. High values of the ratios are also associated with high phosphorus content of the cells. Reasons for the high internal K+ and Mg++ are given in terms of the different coordination chemistry of the four cations and the way in which the concentration ratios might be connected to energy-driven transport then follows. The high external Ca++ is utilized in stabilizing cell walls and in initiating extracellular reactions. Knowledge of eight variables, the concentrations of all four ions both inside and outside cells, is important in an understanding of electrolyte balance.