We have investigated the organization of telomeric TTAGGG)n repeats in the extended DNA loops of chromatin of human and hamster cells by immunofluorescent technique. In humans, telomeric repeats which are predominantly localized at the termini of all the chromosomes, have been found associated with nuclear matrix. This distribution pattern did not alter, even after the removal of 90% of the DNA from the nuclear halos by EcoRI digestion. This suggests that the telomeric sequences are tightly associated with nuclear matrix and hence cannot be solubilized by nucleases. In contrast, in Chinese hamster cells (CHO B11), a major proportion of interstitial telomeric repeats are found in the loop regions, like beads on a string, with attachments to the periphery of the nuclear matrix. Unlike in human cells, EcoRI digestion removed most of the telomeric repeats from the loop regions of Chinese hamster cells. This indicates that intrachromosomal sequences are not associated with nuclear matrix, and this finding has been further substantiated by Southern hybridization of matrix associated and loop DNA fractions of hamster cells with the (TTAGGG)n probe. The organizational differences in the telomeric repeat sequences of Chinese hamster and human cells might be due to their chromosomal location as well as their interaction with nucleoprotein complexes specific for the termini of the eukaryotic chromosomes. Furthermore, the interstitial (TTAGGG)n sequences were found to be more frequently involved in the chromosomal aberrations induced by restriction enzymes. This suggests that the intrachromosomal sites of telomeric sequences behave as hot spots for DNA damage.