The amount of ribosomal DNA (rDNA) was determined quantitatively by RNA-DNA hybridization in the genomes of a mother and her daughter, both with the karyotype 45,XX,t(15q21q). The saturation values found were 0.030% (mother), 0.023% (daughter), and 0.022% for the husband and father of the daughter. A detailed cytogenetic analysis of the short arms of the acrocentric chromosomes of these probands allowed the biochemical results to be interpreted in terms of the size of the individual set of nucleolus organizing regions (NORs) present in each proband. The correlation existing between the biochemical and the cytogenetic findings shows that the amount of rDNA in the human genome is not primarily a function of the number of acrocentric chromosomes, but depends on the individual combination of variant NORs occurring in the human genome.