Abstract. The change of electrokinetic potential (-potential) value of erythrocytes in rat blood has been studied under the influence of electromagnetic irradiation with 41.8 GHz and 42.2 GHz frequencies. It was shown that this physical field with the mentioned frequencies effects on the organisms compared to non-irradiated control ones. Moreover the multiple effect results in more obvious changes of erythrocyte -potential value which indicates that cumulative effect of the factor is observed. It was also shown that the processes start acting in organism directing to weakening of external factor effect.