The use of equilibrium isopycnic CsCl gradients with an initial preformed step in CsCl concentration dramatically reduces the time required to achieve equilibrium conditions. The initial step in CsCl concentration increases the net force on the macrospecies and thus allows the formation of a narrow band at approximately the equilibrium position during the early phase of centrifugation. This type of gradient has been demonstrated to be useful with macrospecies including DNA, RNA, and formaldehyde-fixed ribonucleoprotein particles. © 1969.