Foreign DNA (deoxyribonucleic acid) is part of our environment. Considerable amounts of foreign DNA of very different origin are ingested daily with food. In a series of experiments we fed the DNA of bacteriophage M13 as test DNA to mice and showed that fragments of this DNA survive the passage through the gastrointestinal (GI) tract in small amounts (1-2%). Food-ingested M13 DNA reaches peripheral white blood cells, the spleen and liver via the intestinal epithelia and cells in the Peyer's patches of the intestinal wall. There is evidence to assume that food-ingested foreign DNA can become covalently linked to mouse-like DNA. When M13 DNA is fed to pregnant mice the test DNA can be detected in cells in various organs of the fetuses and of newborn animals, but never in all cells of the mouse fetus. It is likely that the M13 DNA is transferred by the transplacental route and not via the germ line. The consequences of foreign DNA uptake for mutagenesis and oncogenesis have not yet been investigated.