The erythrocyte magnesium content was determined using two different analytical systems (atomic absorption spectrometry and the laser microprobe mass analyzer) with specimens from 18 patients with normal renal function and from 10 patients with chronic renal insufficiency. There was a good correlation (r = 0.73) between the two analytical systems. Additionally, the laser microprobe mass analyzer was used to determine the aluminum content of plasma and erythrocytes in specimens from patients who ingested moderate quantities of aluminum hydroxide for several months. Results showed a significant elevation of the plasma aluminum concentration, but a normal erythrocyte aluminum content. The laser microprobe mass analyzer may be useful in studies of mineral metabolism in humans.