The magnetic properties of free radicals and paramagnetic compounds such as hemoglobin and oxygen are principal factors in evaluating the biological stress of magnetic fields. These compounds exist in high concentrations, i. e. 0.2-10 mM, in human bodies and their environment. Irons in hemoglobin play an important role in the stress induced by magnetic fields. For instance, concentrations of oxygen, nitric oxide and carbon dioxide, which are transported by hemoglobin, are modulated by a magnetic field in the brain, and physiological and biochemical changes, such as accelerated circulation and metabolism, may occur thereafter. Moreover, free radicals, i. e. oxygen, nitric oxide, cerullophasmin-Cu, and transferrin-Fe, may be controlled by an external static magnetic field, and these mechanisms may be related to physiological functions such as circulation. A therapeutic effect of magnetic medical instruments may result, in part at least, from the control of free radicals by magnetic fields and accelerated metabolism described above.