We studied the effect of graphene oxide (GO) nanoparticles on differentiation of human myeloid suppressor cells (MDSC) in an in vitro system. Separated mononuclear cells of healthy donors were induced with cytokines (IL-6 and GM-CSF) into the MDSC phenotype (both polymorphonuclear (PMN-MDSC) and monocyte (M-MDSC) subsets of these cells were taken into account). Pegylated GO nanoparticles (GO-PEG; mean size 569±14 nm, PEG content ~20%) were used. GO-PEG in low concentrations (2.5 and 5 μg/ml) increased the percentage of MDSC in cultures, but reduced their content in high concentration (10 μg/ml). After exposure to GO-PEG (2.5 and 5 μg/ml), the MDSC content increased at the expense of M-MDSC, while the level of PMN-MDSC did not change. The decrease in MDSC levels after exposure to high doses of GO-PEG (10 μg/ml) was due to a decrease in PMN-MDSC. Thus, GO-PEG nanoparticles can oppositely regulate differentiation of MDSC by inhibiting or stimulating differentiation of these cells depending on the concentration.