Silk fibroin (SF)-coated Fe3O4 nanoparticles (NPs) with good superparamagnetism were successfully prepared via a bio-mineralization process at room temperature. Two cell tests revealed that mineralized SF-coated Fe3O4 NPs presented good cytocompatibility for L929 and osteoblast cells and higher cell density after 5 d with high concentrations of SF-coated Fe3O4 NPs (up to 0.5 mg/mL). These resulted from SF surface coating on NPs, nano-surface morphology and iron ion release of Fe3O4 NPs. The mineralized SF-coated Fe3O4 NPs could be envisioned for various bone orthopedic and therapeutic applications, in which SF-coated NPs location is controlled through an external magnetic field to promoted bone growth. (C) 2011 Elsevier Inc. All rights reserved.