A simple method for the determination of ionized magnesium in erythrocytes using a potentiometric analyzer Microlyte 6 (Kone, Finland) was investigated. A set of aqueous solutions containing electrolytes at the same concentration as in erythrocytes in Tris/TES buffer (pH 7.2) was used as standards. The erythrocyte cell membranes were destroyed using three different methods: (A) HCl solution of pH 3.5, (B) dry ice–acetone mixture, (C) ultrasound. The result did not differ significantly between methods B and C (Student’s test, α=0.01). The results were compared with those obtained using the 31 P nuclear magnetic resonance method ( 31 P NMR). The dissociation constant of MgATP was calculated from the 31 P NMR results. The results obtained from potentiometry and 31 P NMR did not differ significantly (Student’s test, α=0.01). Total magnesium concentration was determined using atomic absorption spectrometry (AAS).