Abstract Some properties of the repeated sequences in the DNA of the human genome have been investigated. The human genome consists of 7 × 10 9 base pairs and about 35% of these are repeated sequences. The repeated sequences were divided into families, or groups of families having different reassociation rates. The most rapidly-renaturing repeated sequences constitute 10% of the genome and appear to be closely spaced, possibly in tandem arrangement, while the slowly-reassociating repeated sequences are dispersed. In addition, the rapidly-reassociating sequences were fractionated with respect to the G + C content by thermal elution chromatography. Repetitious sequences, both fast- and slow-reassociating, were found in every arbitrary temperature segment. This indicates the heterogeneity of the repeated sequences since they apparently occurred in the entire spectrum, from very A + T-rich to very G + C-rich varieties. In situ DNA-[ 3H]RNA hybridization experiments were performed with human cells and [ 3H]RNA complementary to the various human repetitious DNA fractions. The results suggest that (a) unlike the case of mouse where essentially all centromeric heterochromatins are composed of one DNA family, human heterochromatin is composed of a variety of DNA families, and (b) mapping human chromosomes by nucleic-acid hybridization appears feasible.