Human DNA was fractionated by centrifugation in Cs2SO4 density gradients containing 3,6-bis(acetatomercurimethyl)dioxane (BAMD). Fractions were investigated in their analytical CsCl profiles and a number of specific sequences were localized in them. The results so obtained led to an improved understanding of the organization of nucleotide sequences in the human genome, as well as to the discovery that a class of DNA having a very high G + C content and not represented in the mouse genome, is particularly rich in genes and interspersed repetitive sequences.