The human SWI/SNF complex remodels nucleosome structure in an ATP-dependent manner, although the nature of this change has not been determined. Here we show that hSWI/SNF and ATP generate an altered nucleosomal structure that is stable in the absence of SWI/SNF. This product has an altered sensitivity to digestion by DNAse, restriction enzymes, and micrococcal nuclease, and an increased affinity for GAL4. It has the same protein composition but is approximately twice the size of a normal nucleosome. Incubation of the altered nucleosome with hSWI/SNF converts this structure back to a standard nucleosome in an ATP-dependent process. These results suggest that hSWI/ SNF acts by facilitating an exchange between normal and altered, more accessible, nucleosome conformations.