A huge magnesium isotope effect on the rate of enzymatic synthesis of adenosine triphosphate (ATP) has recently been reported. This indicates that the reaction involves an intermediate ion-radical pairs and should be sensitive to external magnetic fields. We have calculated the rate of ATP production based on a previously proposed molecular reaction mechanism. The calculated maximum isotope effect is in agreement with experiment. Magnetic field is predicted to increase the rate of ATP formation by 60% for samples enriched by magnetic magnesium (Mg-25) while decreasing the rate by 9% for naturally occurring samples. (c) 2006 Elsevier B.V. All rights reserved.