When human erythrocytes were irradiated in vitro with lower energy soft x-ray, their EPM dropped markedly, This is of similar effect as treated with hard x-ray. Within the range of 43.9-1754.0 rad, the drop of erythrocyte EPM varied in proportion to irradiation dosages, When erythrocytes underwent irradiation of 43.9 and 1052.4 rad, its EPM changed with the lapse of time, At 4 hrs after irradiation, the erythrocyte EPM dropped to its minimum, then gradually recovered. For samples irradiated with 43.9 rad, their EPM recovered to its original level 24 hrs later, but not for samples with 1052,4 rad irradiation.,When the erythrocytes were suspended in their own plasma, their EPM was lower than in normal saline, but when irradiated plasma was added (1:1) into saline suspension of normal erythrocytes, no EPM change was induced. However, when irradiated erythrocytes were suspended in normal saline, their EPM dropped Markedly. Furthermore, the dropped EPM might be recovered if CDP-choline was added into irradiated erythrocyte suspension.