The inheritance of nucleolus organizer regions (NORs) was investigated by examining the degree of silver-staining in individual acrocentric chromosomes in two successive generations. The study was undertaken in six Down's syndrome children and their respective parents. Quinacrine fluorescent polymorphisms were used to identify individual acrocentrics and to determine which of the child's acrocentrics were informative as to parental homologue of origin. Of the 66 acrocentrics in the six children, 31 were informative. The correlation between the degree of silver=staining in the child's chromosomes and the respective parental chromosomes of origin was highly significant (P less than 0.001), with a correlation coefficient of 0.90. The results suggest that the degree of Ag-AS staining is characteristic for a particular chromosome and that this characteristic is an inherited property.