The aim of this study was to determine the biocompatibility and potential toxicity of apatite-coated magnetite nanoparticles. The in vitro biocompatibility with human red blood cells was evaluated, not hemolytic effects were found at concentrations lower than 3 mg/ml. For the in vivo study, Balb/c mice were used. The animals were injected intravenously or intraperitoneally, the doses ranged from 100 to 2,500 mg/Kg. All the injected animals showed normal kidney and liver function. No significant changes were found in the body weight, the organs weight and the iron levels in liver due to the administration. In conclusion, apatite-coated magnetite nanoparticles did not induce any abnormal clinical signs in the laboratory animals. The results demonstrated that apatite-coated magnetite nanoparticles of 8 ± 2 nm in size did not have hemolytic effect in human erythrocytes and did not cause apparent toxicity in Balb/c mice under the experimental conditions of this study.