The operation of the Mg2+ dependent Na+K+ ATPase “pump” located in the plasma membrane has generally been assumed to be responsible for maintaining a major portion of the cell’s asymmetric ion distribution by pumping cellular Na+ out and extracellular K+ in. This model assumes the “pump” is the primary determinant of K+ content, that the mM concentrations of Na+ and K+ exist in a free state and are therefore immediately available to the pump. If this is the case, analysis of ion content within the cytoplasm and nuclear compartments should show a Na+ content that is negatively correlated to the K+ content and multiple regression analysis of ion content should show Na+ concentration to be a major predictor of K+ content. However, recent experiments are beginning to show that K+ is not necessarily in a free state in the cell but may instead be bound to protein and nucleic acids, von Zglinicki et al. (1986) found no significant concentration differences between the decondensed chromatin within the nuclei and the cytoplasm, but found K+ concentrations in condensed chromatin to be higher than was consistent for ions free in solution. Furthermore, Kellermayer et al. (1984) found that disruption of lymphocyte membrane integrity did not result in the immediate loss of K+ from the nuclei as expected if K+ ions were free in solution. The results suggest that K+ content is associated with an essentially immobile component such as proteins or nucleic acids. We were interested in determining what the state of the K+ ion was in the nucleolar and condensed chromatin compartments of the cell. We wished to specifically test if the K+ ion concentration were negatively correlated with Na+ ion concentration with Na+ being the major predictor for K+ content as the “pump” model would require, or if, P and S content would be the major predictors of K+ were K+ bound to proteins (the major source of total cellular sulfur) or to nucleic acids (the major source of cellular phosphorus).