The pattern of association of acrocentric chromosomes was examined in ten and 
five carriers of a 15/21 and a 13/14 Robertsonian translocation, respectively, 
and was compared with that of the same numbers of relatives with normal 
karyotypes. In the carriers of 15/21 translocation, the number of large 
associations (involving more than two acrocentrics) and the association 
frequencies for individual acrocentric chromosomes, were significantly higher 
than in the control group. The mean number of associations of the single 
homologs of the translocation chromosomes was much higher than that of the other 
acrocentrics. In the carriers of 13/14 translocations, only the association 
frequency for chromosome 13 was higher than in the normal relatives. The 
uninvolved chromosomes homologous to those involved in translocation showed an 
insignificant increase in associations in comparison with the other 
acrocentrics. These results suggest that some mechanism within the cells 
compensates for the effect of missing acrocentrics or of acrocentrics lacking 
NORs on the number of associations. The possible relations of this phenomenon to 
the activity of the nucleolus organizing regions are discussed.