Less than 1% of the total body magnesium is present in the peripheral blood. We developed an assay to determine the magnesium content of blood mononuclear cells as an index for intracellular magnesium and/or total body magnesium status. The blood magnesium status (plasma and red cell concentrations and mononuclear blood cell content of magnesium) was assessed in patients with parathyroid disease. The results show that patients with primary hyperparathyroidism have a decrease in the concentration of magnesium in red blood cells and in the content of magnesium in mononuclear blood cells compared with normal individuals. Patients with chronic hypoparathyroidism showed the opposite results. There was no significant change in plasma magnesium concentration among the groups.