Nanoparticles (NPs) are materials with one dimension in the range of 1-100 nm. The toxicity of NPs remains widely unknown and still poses concerns, due to the peculiar characteristics of materials in the nano-size range. We analyze the toxicity of seven NPs ((Fe2O3, Fe3O4, Sb2O3, Au, TiO2, Co, and Ag) on primary cultures of human hematopoietic progenitor cells from the bone marrow of healthy donors with CFU assays, and show that antimony oxide (Sb2O3) NPs and cobalt (Co) NPs have a toxic effect, while the other NPs have no effect at the tested concentrations (5, 25 and 100 microg/ml). While Co NPs suspension is toxic to both erythroid and granulocytic-monocytic precursors, Sb2O3 NPs at 5 microg/ml are specifically toxic to erythroid colony development, suggesting a highly selective type of toxicity. With liquid culture assays we show that Sb2O3 NPs impair the proliferation of erythroid progenitors, while no toxic effect is observed when Sb2O3 NPs are added during erythroid differentiation. CFU assays and liquid culture assays on seven human cell lines of hematopoietic origin (K562, HL-60, CEM, CEM-R, Thp-1, Jurkat, and Molt-4) show that, contrary to what observed on primary cultures of bone marrow progenitors, Sb2O3 NPs have no toxic effect on proliferation of any of the cell lines, raising concerns about the use of immortalized cell lines for nanotoxicology tests.