Recently, Cameron et al. ('79) published measurements of intracellular solute amounts (expressed as mmoles per kilogram dry cell solids) obtained by energy dispersive electron probe microanalysis in different rodent tissues. In this communication, I wish to compare Cameron et al.'s ('79) erythrocyte values of Na, K, and Cl with those I have made using more conventional techniques of elemental analysis. This comparison is necessitated by Cameron et al.'s ('79) observation of extremely high intracellular sodium levels. If their findings are accurate, the possibility of a polymorphism with respect to intracellular Na levels therefore presents itself. If a polymorphism between different inbred strains of mice exists, and if, as in ruminants, this trait has a genetic basis, an examination of the genetic aspects of the control, differentiation, and the ultimate expression of the ion transport mechanisms responsible would undoubtedly provide insight into the molecular basis as well as the adaptive dynamics of transport systems in general.