Induced pluripotent stem (iPS) cells have been generated from mouse and human somatic cells by introducing Oct3/4, and Sox2 with either Klf4 and c-Myc or Nanog and Lin28 using retroviruses or lentiviruses. Patient-specific iPS cells could be useful in drug discovery and regenerative medicine. However, viral integration into the host genome increases the risk of tumorigenicity. Here we report the generation of mouse iPS cells without viral vectors. Repeated transfection of a single plasmid containing the cDNAs of Oct3/4, Sox2, and Klf4, together with a c-Myc expression plasmid, into mouse embryonic fibroblasts resulted in iPS cells without evidence of plasmid integration, which produced teratomas when transplanted into mice and contributed to adult chimeras. The production of virus-free iPS cells, albeit from embryonic fibroblasts, addresses a critical safety concern for potential use of iPS cells in regenerative medicine.