The change of cell membrane charge was induced by soft X-ray. This change was seen not only on the erythrocytes, but also on the lymphocytes and platelets. It was revealed by the drop of EPM (electrophoretic mobility). Within the low dose range, this change of the charge was temporary. At 4 hrs after irradiation, the EPM dropped to its miniumm; 24 hrs later, it returned to its original level. The extent of EPM depended on irradiation dosage.,The lymphocytes are a group of complicate cells According to the cell EPM, they could be divided into two subgroups: T cell and B cell. The EPM of normal T cells was faster than that of B cells. After T and B lymphocytes were irradiated by soft X-ray, the EPM of both subgroups became slower, the peak of frequency distribution lowered down and the peak width extended. The platelets are homogeneous particle population and their EPMs appeared to be identical.,When the concentrated samples of platelets after irradiation were suspended in unirradiated plasma, the drop of EPM was much faster than that of the unirradiated platelets suspended in the irradiated plasma. When the irradiated platelets were suspended in phosphate buffer, their EPM were much lower than that of the same platelets suspended in their own plasma. If the platelets were suspended in their own plasma and then irradiated with doses up to 2000 rads, their EPM drop could recover after irradiation; but the recovery was not seen in cases of irradiating the platelets suspended in phosphate buffer. Based on the above results, it can be considered that there are some factors in the plasma which protected the platelets from radiation damages and promoted the damaged platelets to recover, and besides, the drop of EPM can be prevented by SOD (superoxide dismutase) .