The determination of blood mononuclear cell magnesium content may be a predictor of total body magnesium status. Separation techniques employing Ficoll-Hypaque have been used for this assay; however, Hypaque contains a significant concentration of iodine that may be toxic to the cell membrane and affect the results. We developed a different separation technique using arabinogalactan (Stractan) at concentrations of 13.0 and 17.7%. The blood from 16 normal volunteers was divided and assayed for mononuclear cell magnesium content using both methods. The results (mean +/- standard deviation) with the Ficoll-Hypaque method were 83.6 +/- 30.8 fg/cell, and with the arabinogalactan method they were 78.4 +/- 25.5 fg/cell. A paired sample t test comparing the results between the two methods gave a t value of 1.05 (P greater than .3). The purest population of blood mononuclear cells was obtained with the Ficoll-Hypaque method. Thus, these two separation materials do not have a significant effect on the blood mononuclear cell magnesium content, and the better separation of cells was obtained with Ficoll-Hypaque.