Repressive chromatin structures need to be unravelled to allow DNA-binding proteins access to their target sequences. This de-repression constitutes an important point at which transcription and presumably other nuclear processes can be regulated(1,2). Energy-consuming enzyme complexes that facilitate the interaction of transcription factors with chromatin by modifying nucleosome structure are involved in this regulation(3-5). One such factor, nucleosome-remodelling factor (NURF), has been isolated from Drosophila embryo extracts(4,6,7), We have now identified a chromatin-accessibility complex (CHRAC) which uses energy to increase the general accessibility of DNA in chromatin. However, unlike other known chromatin remodelling factors, CHRAC can also function during chromatin assembly : it uses ATP to convert irregular chromatin into a regular array of nucleosomes with even spacing. CHRAC combines enzymes that modulate nucleosome structure and DNA topology. Using mass spectrometry, we identified two of the five CHRAC subunits as the ATPase ISWI, which is also part of NURF6,8, and topoisomerase II, The presence of ISWI in different contexts suggests that chromatin remodelling machines have a modular nature and that ISWI has a central role in different chromatin remodelling reactions.