Arrighi and Hsu (1971) have described a method of differential staining of chromosomes based on the localization of repetitive DNA associated with heterochromatin. We have employed this method in an attempt to identify human chromosomes, especially the G group, Nos. 21 and 22 and the Y. Metaphases from a patient with 47,XY,21+ were examined. In addition to the Y chromosome, which shows dense staining in its long arm, three of the five G-group chromosomes show centromeric regions that stain much more densely than those of the other two. This indicates that it is possible to differentiate chromosome 21 from 22 and the Y. The possibility of identifying other chromosomes by similar differences is a likely one.