We evaluated three methods (two indirect and one direct) for determining the magnesium (Mg) and zinc (Zn) content of erythrocytes, to compare methodologic differences and to establish a method suitable for use in field studies. For the indirect methods, erythrocytes in whole blood were lysed by adding either de-ionized water (I) or nitric acid, 2 mol/L (II). For the direct method (III), erythrocytes were isolated by density centrifugation, washed, then digested in concentrated HNO3. Mg and Zn concentrations were measured by atomic absorption spectrophotometry in plasma and whole blood for the indirect methods, and in the pellet for the direct method. Hematocrit and hemoglobin were measured, and erythrocytes were sized and counted on all samples. Within-run CVs for the three methods ranged from 2.2% with method III for Mg to 5.4% with method I for Zn. CVs for reproducibility of the analytical procedures ranged from 2.6% with method II for Zn to 4.2% with method I for the two cations. Analytical recoveries of added Mg and Zn ranged from 93.8 to 104.7%. When values for the three methods were compared, those by method I were significantly (p less than 0.05) lower than those by methods II and III. Values obtained by method II were 100.1% for Mg and 102.4% for Zn of those by the direct method. Thus, the indirect method with 2 mol/L HNO3 lysing solution provides a reproducible, reliable, accurate, and simple technique for measuring Mg and Zn in erythrocytes.