Five human clones containing genomic regions of polydA have been isolated by their ability to form intermolecular triple helices with agarose cross-linked polyU. All of these clones contain Alu repetitive DNA sequences. End-labelled DNA fragments containing these sequences have been successfully reconstituted onto nucleosome core particles by salt exchange. The structure of these has been examined by digesting with DNase I, hydroxyl radicals or diethylpyrocarbonate. DNase I cleavage of the polydA tracts is poor in the free DNA but is markedly enhanced at certain positions when complexed with nucleosome cores. Phased digestion patterns are observed which continue through the (A) _n blocks and reveal an average helical periodicity of about 10 base pairs. The distance between adjacent maxima varies between 8–12 base pairs, suggesting that the exact helical repeat is not necessarily constant. One fragment containing the sequence (TA) ₁₁ T ₃₄ reveals a 12 base pair repeat within the (AT) _n region. A pUC19 polylinker fragment containing a block of A ₆₉ .T ₆₉ cloned into the Smal site could also be reconstituted onto nucleosome cores and reveals the same phased DNasel digestion pattern. The DNase I cleavage pattern is not identical at each of the maxima, suggesting that the structural distortions imposed by the core particles are not constant along the DNA.