Magnetite nanoparticles pre-coated with dodecanoic acid and ethoxylated alcohol (DE) were used to obtain a physiologically stable magnetic fluid (DE-MF) sample. Three different doses of DE-MF were intraperitoneally applied to mice. Blood and peritoneum cytometry and micronucleus test were performed for 1-21 days after injection to investigate the DE-MF toxicity. Changes in cell population, peritoneum inflammation, and potential DE-MF genotoxic action were all time and dose dependent. At the lowest dose (5x10 1 5 particles/kg), DE-MF seems to be useful as a drug precursor with both diagnostic and therapeutic values.