One hour after intraperitoneal injection of DMN-3H into control mice the highest specific activity was in the supernatant fraction followed by mitochondrial, microsomal, and nuclear fractions. Although 50–80 percent of the radioactivity was removed by washing with acid and lipid solvents in the former three fractions, little radioactivity was extracted from the nuclear fraction by the washing. This suggests that in the nucleus most of the radioactivity was bound firmly to either nucleic acids or proteins, whereas in other fractions much of the radioactivity existed in an unbound state.