It is clear now that although different ionophores for ionized Mg (IMg2+) have been designed by several groups, each of these has a distinctly different KMgCa. In view of this, it is important to determine whether each of these ion selective electrodes (ISE's) yield identical results for IMg2+ in sera from healthy and diseased humans. Using such an approach, we determined, in a blinded-and random manner, IMg2+ with both the NOVA and KONE ISE's for IMg2+ in two independent laboratories. No significant differences were found either for sera from healthy human volunteers or diseased patients. We did, however, note several interesting findings: 1. randomly, selected hospitalized patients exhibit a much higher incidence of abnormalities for IMg2+ (57-71%) than that noted previously for total Mg (TMg) measurements; and 2. coronary heart disease, rectal cancer and multiple sclerosis patients exhibit extracellular deficits in ionized free Mg.