An exposure of mice to a constant magnetic field (CMF) of 9.9, 25.4 and 39.4 kersted produced changes in the leucocyte and reticulocyte counts and caused no changes in the red blood cell count and hemoglobin. The changes noted during and after the exposure were of a cyclic pattern. During a 24-hour exposure of animals to a CMF of 39.4 kersted a short-term increase in the reticulocyte and leucocyte counts and a concomitant decrease of the neutrophil and lymphocyte counts were noted. Afterwards the reticulocyte count decreased and increased again by the end of the exposure. Following a 3-hour exposure to CMFs of the above intensities the changes persisted for 20-35 days. Normally the parameters increased during the 1-2 days of the exposure and decreased thereafter. The level of changes was not correlated with the CMF intensity. The increase in the reticulocyte count during and exposure to a CMF of 39.4 kersted was identical to that during an exposure to a CMF of I kersted.