Calcium phosphate nanoparticles were prepared by precipitation from water and were then functionalized by DNA. These particles are taken up by living cells and function as gene transfer agents, i.e., the DNA is brought into a cell's nucleus and is incorporated there into the cell's genome (transfection). DNA which encodes for enhanced green fluorescent protein leads to green fluorescence of successfully transfected cells. By adding the red-fluorescing marker tetramethylrhodamine isothiocyanate-bovine serum albumin (TRITC-BSA) to the nanoparticles, their pathway into the cell and within the cell could be followed by fluorescence microscopy. A clear correlation between the uptake of nanoparticles and the efficiency of transfection was found. Aggregates of DNA/TRITC-BSA alone were not able to enter the cells, i.e., the inorganic nanoparticles are necessary as a carrier through the cell membrane.