We report an alternative method for electrophoretic separation of large DNAs using steric confinement between solid walls, without gel or obstacles. The change of electrophoretic mobility vs confinement thickness is investigated using fluorescence video microscopy. We observe separation at small confinement thicknesses followed by a transition to the bulk behavior (no separation) at a thickness of about 4 mu m (a few radii of gyration for the studied DNA chains). We present tentative explanations of our original observations. (C) 2007 Elsevier Inc. All rights reserved.