Nanotechnology in biomedical research is an emerging and promising tool for different purposes, like high-resolution medical imaging, diagnostics, and targeted drug delivery. Although some experimental efforts focused on determination of the cytotoxicity of nanoparticles (NPs) are present, there are many controversial results. In this study, chitosan (CS)- and poly(acrylic acid) (PAA)-coated iron oxide nanoparticles (IONPs) were used to investigate the possible cytotoxicity of coated IONPs on model mammalian cell line SaOs-2 by evaluating cellular viability and membrane integrity. Increasing concentrations of IONPs increased the cytotoxic effect on SaOs-2 cells with both CS- and PAA-coated IONPs. Cell viability on day 3 was as low as 40% and 48% at 1000 ?M concentration for PAAand CS-coated IONPs, respectively, in 1% FBS-supplemented media. Cytotoxicity determined by the released lactate dehydrogenase (LDH) was as high as 163% with 1000 ?M concentration of CS-IONPs, while there was no significant change in LDH release with PAAcoated IONPs at any concentration. This study reveals that IONPs coated with a biocompatible polymer, which are usually assumed to be nontoxic, show cytotoxicity with increasing concentration and incubation time. The cytotoxicity of nanoparticles intended for biomedical purposes must be evaluated using more than one approach.