The aim of the paper was to estimate in vitro the effect of electromagnetic field produced by mobile phones on the activity of superoxide dismutase (SOD-1) in human blood platelets. Suspension of blood platelets exposed to the electromagnetic field of 900 MHz frequency for 1, 3, 5, 7 minutes. Our studies demonstrated that microwaves produced by mobiles significally depleted the activity of SOD-1 after 1, 5, 7 min. of exposition and increased after 3 min. nn comparison wit control test. On the ground of our results we conclude that oxidative stress after exposition to microwaves can be the reason of many disadvantageous changes in cells and may cause many systemic consequences in human organism.