The acceleration of H202 decomposition induced by catalase and the dimer complex [Fe3+(EDTA)]2 has been observed in a constant magnetic field. The effect increases with the field increasing up to 8000 Oe, reaching 20 +/- 5% and 24 +/- 5% for catalase and [Fe3+(EDTA)]2 respectively. The results are discussed within the hypothesis of a one-electron reaction mechanism using the models developed specially to explain the magnetic effects in radical reactions. It is supposed that the stage which is affected by the magnetic field is the electron transfer coupled with Fe3+O./2 paramagnetic species. The interpretation proposed does not exclude an alternative possibility of the magnetic effects during the electron transfer to two iron atoms by a two-electron (synchronous) mechanism.