In order to reveal the biocompatibility of Fe3O4 nanoparticles and bipolar surfactant tetramethylammonium 11-aminoundecanoate cytotoxicity tests were performed as a function of concentration from low (0.1 mu g ml(-1)) to higher concentration (100 mu g ml(-1)) using various human glia, human breast cancer and normal cell lines. Cytotoxicity tests for human glia (D54MG, G9T, SF126, U87, U251, U373), human breast cancer (MB157, SKBR3, T47D) and normal (H184B5F5/M10, WI-38, SVGp12) cell lines exhibited almost nontoxicity and reveal biocompatibility of Fe3O4 nanoparticles in the concentration range of 0.1-10 mu g ml(-1), while accountable cytotoxicity can be seen at 100 mu g ml(-1). The results of our studies suggest that Fe3O4 nanoparticles coated with bipolar surfactant tetramethylammonium 11-aminoundecanoate are biocompatible and promising for bio-applications such as drug delivery, magnetic resonance imaging and magnetic hyperthermia.