Less than 1% of the total body magnesium is present in the peripheral blood. We developed and assay to determine the magnesium content of blood mononuclear cells which did not correlate with either the plasma or erythrocyte magnesium concentrations. The blood mononuclear cell magnesium concentration may be a better predictor of intracellular magnesium content and/or total body magnesium status than either plasma or erythrocyte magnesium concentrations.