Existing procedures for the preparation of rat liver plasma membranes are time consuming and generally produce low yields. A method is described in which a rat liver homogenate low-speed pellet is fractionated on a self-forming Percoll gradient. Plasma membranes can be removed from the gradient in a high yield along with much of the DNA in the liver homogenate. A second Percoll step performed in the presence of a low concentration of calcium ions separates the DNA from the plasma membranes. The final membrane fraction has high specific activities of marker enzymes with little contamination with microsomal, mitochondrial, Golgi, or lysosomal markers.