Chinese hamster DON cells with 5-bromodeoxyuridine (BrdU)-substituted chromosomes were ultraviolet (UV)-exposed and processed for in situ detection of induced DNA breaks under electron microscopy. For this purpose, UV-induced breaks were amplified by an exonuclease III digestion to obtain single stranded DNA motifs which could hybridize with oligonucleotides of random sequences. These reannealed motifs could be used as primers which were extended by the Klenow polymerase, incorporating biotinylated-dUTP that was detected by a gold-tagged streptavidin. After processing, the chromatid whose DNA was BrdU-substituted in one strand showed a higher electron density than the chromatid substituted in both strands. In contrast, the unifilarly substituted chromatid showed about twice the labelling of DNA breaks as the bifilarly substituted one. This result could be the consequence of a greater loss of chromatin tracts in the bifilarly substituted chromatid, as implied by an X-ray microanalysis which showed that the amount of phosphorous lost by the bifilarly substituted chromatid was higher than that of the unifilarly substituted chromatid.