Montmorillonite, a bioinert clay mineral, was examined as a novel vector for an oral gene-delivery system. The complex of montmorillonite and plasmid DNA encoding the enhanced green fluorescent protein (EGFP) gene was prepared at various weight ratios, and then transfected into cultured intestinal epithelial cells (IEC-6) in vitro. The EGFP gene was clearly transcribed when the transfection was performed using the montmorillonite–plasmid complex at a weight ratio of 0.05:1. In contrast, no gene expression was detected by reverse transcription-polymerase chain reaction (RT-PCR) analysis when the transfection was performed with naked plasmid. Various plasmid preparations were given orally to mice, and the gene expression in the stomach and small intestine was examined by RT-PCR. Although no gene expression was detected in the mice receiving an oral administration of naked plasmid or polyethyleneimine–plasmid complex, the EGFP gene complexed with montmorillonite was expressed in the small intestine. These results indicate that montmorillonite protected the plasmid DNA from the acidic environment in the stomach and DNA-degrading enzymes in the intestine, and successfully delivered it into cells of the small intestine.