PEMF treatment was studied in 10 patients with multiple sclerosis and 10 patients with migraine. In both patients' groups a single treatment induced a significant rise of yeast particle uptake by blood granulocytes. The percentage of phagocytizing cells was increased in migraine patients only. In both patients' groups 20 PEMF treatments caused a reduction of particle uptake, whereas the percentage of phagocytizing cells remained unchanged. In migraine patients the opsonic capacity of serum and the mean cell volume of erythrocytes, lymphocytes and granulocytes were initially reduced, but increased during the course of 20 PEMF treatments. The biphasic changes of cell volume and phagocytic activity are interpreted as a result of counter-regulation of the organism in response to the primary PEMF effect.