The integration of hepatitis B viral sequences in the human hepatoma Alexander cell line has been investigated after fractionation of the cell line DNA by centrifugation in a Cs2SO4/BAMD (3,6-(bis-acetato mercurimethyl) dioxane) density gradient. Eight out of nine integrated viral sequences were localized in DNA component H3, which only represents 4% of the human genome and matches the base composition of HBV sequences. These results indicate a targeting and/or a higher stability of the latter in a specific, small compartment of the host genome.