The authors examined comparatively the frequencies of genomic, counted and structural chromosomal aberrations and associations of acrocentric chromosomes (AAC) in lymphocytic cultures from 35 phenotipic healthy newborns and 15 clinically healthy persons over 60 years of age. Polyploidy was found in 0,16% of the cells in the newborns and in 0,47% of the cells of the adults (0,1 greater than P greater than 0,05). Aneuploidy was established in 0% of the cells of the newborns and in 5,7% of those of the adults (P less than 0,05). In the hyperploid karyotypes 1,3% of the cells of the adults the additional chromosomes were of group C and D. Structural aberrations were observed in 0,58% of the cells of the newborns and in 3,6% of those of the adults (P less than 0,05%. Chromosomal and chromatoid breaks with or without the presence of fragment predominated. Cells with AAC in the newborns were 61,7% against 81,4% in the adult persons (P less than 0,001). The number of the associating chromosomes increased with advancement of age--19,2% in the newborns and 31,9% in the adults (P less than 0,001). In the newborns 21,5% of the chromosomes from group G participated in associations and 17,7% of chromosomes from group D; in the adults both groups were equally engaged--31,3% of the chromosomes G and 32,4% of the chromosomes D.