The effect of de magnetic fields of the order of 0-4000 G on the oscillating peroxidase-oxidase (PO) reaction was studied. Specifically the effects of magnetic fields on the amplitude of simple periodic oscillations and on complex dynamic states were investigated. Magnetic fields induce a biphasic change in oscillation amplitude. At field strengths up to 1500 G, the amplitude decreases gradually by 11%. At higher field strengths the decrease in amplitude is less, and it vanishes at field strengths of 4000 G and higher. The effect of magnetic fields on complex dynamics is generally to shift the dynamics from one state to a neighboring state. Numerical simulations of a detailed model of the PO reaction support the hypothesis that the magnetosensitive step is the reduction of compound II to ferric peroxidase or the reduction of compound I to compound II or both.