We studied the effects of several heparin anticoagulants on ionized calcium and ionized magnesium measurements. Venous blood from 24 healthy donors was drawn into 20-ml syringes without an anticoagulant, and immediately analyzed for control results. They were then transferred to completely fill and half-fill each of four brands of syringes, and an evacuated non-anticoagulated blood-collection tube. All were analyzed for ionized calcium and magnesium. In some cases, either saline or a Ca/Mg solution was added to increase the range of concentrations. The final concentrations ranged from 0.94 to 2.09 (ionized calcium) and from 0.41 to 1.06 mmol/l (ionized magnesium). In full syringes, mean changes in ionized calcium or magnesium were <0.01 mmol/l over a range of concentrations usually encountered. The only definite effect was noted in half-full Martell syringes, where ionized calcium was lower by 0.028 mmol/l. At very high ionized calcium concentrations (>1.95 mmol/l) and ionized magnesium concentrations (>0.95 mmol/l), a few changes were noted of little clinical significance. We conclude that all the syringes evaluated have no clinically significant effects on ionized calcium or ionized magnesium concentrations when filled to capacity. When half-full, a few changes of marginal clinical significance were noted. All these effects were less than observed in serum, which is apparently affected by sample processing and/or clotting.