In a sample of 63 like-sex dizygotic twins, red blood ceE and plasma magnesium and red blood cell zinc concentrations were analyzed for linkage to each of 23 genetic systems by estimating correlation between proportion of genes identical by descent and biological resemblance for the trait. The results suggest possible linkage of red blood cell magnesium with the HLA locus and of red blood cell zinc with the GLO1 locus. However, studies applying more powerful tests are needed to confirm such conclusions.