Previous work suggested that chromosomes 21 and 22 could be distinguished by fluorescent labelling of the chromosomes with quinacrine mustard (QM). This suggestion was confirmed in the present study of patients with Down's syndrome since it was found that in most cases three of the five G chromosomes had a clearly different distribution pattern from the remaining two. The pattern typical of chromosome 21 displays a particularly bright fluorescence in the middle of the long arm, whereas the fluorescence of chromosome 22 is less intense and more equally distributed along the length of the chromosome. Individual patterns of fluorescence have been observed in chromosome 21. Some of the implications of the findings are discussed.