Total and free magnesium contents were determined by atomic absorption spectrophotometry and 31P nuclear magnetic resonance on red blood cells of healthy blood donors of known HLA groups. A statistically significant correlation was found between free and total magnesium contents (r = 0.67, P less than 0.02). The previously described low total magnesium contents in HLA-B35+ subjects were associated with low free Mg2+ contents. Therefore, total and free magnesium and HLA-associated genetic factors are closely related.