The turn of a polarization plane of the light disseminated at app. 180° by the erythrocytes suspension in physiological solution, diluted with a buffer solution is found in external magnetic field. Verde constant and coercive force are defined. Results are interpreted on the basis of a model according to which magnetic properties of blood are caused by the magnetic moment of the heme, being a part of a myoglobine molecule and containing a bivalent iron ion in the porphyrine ring.