Objective: to detect changes in the parameters of the structure and mechanical properties of the surface layer of lymphocytes in the blood of a rat after a single action (30 min and 2 h) of an extremely low frequency electromagnetic field (ELF EMF) on the rat`s whole blood in vitro.Material and methods. We exposed the whole blood of a Wistar rat (4.5 months) to the effect of ELF EMF (50 Hz, B = 0.79 mT), generated by an experimental setup based on a Helmholtz coil. Lymphocytes were isolated from the blood using the Histopaque-1077 density gradient sedimentation method, then were placed on the surface of glass slides and fixed with glutaraldehyde, and finally were dried. We performed atomic force microscopy (AFM) using the atomic force microscope Bioscope Resolve in the PeakForce QMN mode in air.Results. On the basis of the results of the analysis of the AFM images, we classified the lymphocytes into several morphological types depending on the size of the lamellopodium. We analyzed the mechanical properties of the surface areas sized 250 nm × 250 nm for lymphocytes with the lamellopodium sized 400–1100 nm. The analysis found an rise in the elastic modulus of the lymphocyte surface after a short-term action (30 min) of ELF EMF. When the time of the ELF EMF exposure was increased up to 2 hours, the elastic (elastic modulus) and adhesive properties (the adhesion force between the AFM probe tip and cell surface) of the lymphocytes were significantly weakened.Conclusion. For the first time using the AFM method we have detected nanoscale changes in the structure and mechanical properties of the surface of the rat`s lymphocytes after a short-term action of ELF EMF (50 Hz, 0.79 mT, 30 min), which are indicative of the activation of the functions of lymphocytes as an immune system component in response to the short-term exposure of ELF EMF.