A method is described in which RNA is banded in a CsCl density gradient under conditions where solid CsCl is present at the bottom of the cell. The density of the solution at the interface between solution and solid salt serves as a reference for the calibration of densities anywhere in the liquid column. The buoyant density of MS2 RNA, banded in aqueous CsCl at 40°;, was determined in this way and found to be in agreement with the known value for this molecule. Aqueous CsCl solutions are not sufficiently dense to band RNA at room temperature. It is demonstrated that substitution of 2H2O for H2O as solvent for CsCl makes the banding of RNA at 25° possible.