Chromosomes 1, 3, 4, 15, and 16 show lateral asymmetry after replication in the presence of BrdU and staining with Giemsa at pH 10.4. Lateral asymmetry is thought to result from an unequal interstrand distribution of thymine in regions of repetitive DNA. Compound lateral asymmetry in 1 may be due to the presence of two segments of DNA differing in base composition and with different strands rich in thymine. Polymorphism for lateral asymmetry is observed in the AT-rich quinacrine-positive centromeric regions of 3 and 4, even though none of the four satellite DNAs are demonstrable by in situ hybridization there. Other workers have shown that a satellite fraction isolated using Hoechst 33258 and an EcoRl restriction fragment of human satellite DNA map on 3. There is no evidence for the presence of satellite DNA on 4. Since 20% of the DNA in the human genome reassociates at a C0t of ≤ 1 and only 4% is satellite DNA, 4 may have repetitive nonsatellite sequences; on the other hand the lack of an appropriate satellite probe may be the limiting factor. Lateral asymmetry in the Y has been demonstrated using the Hoechst method by other workers but cannot be observed with BrdU-Giemsa staining at pH 10.