Immunolatex spheres, originally developed as visual markers for scanning electron miscroscopy, were employed as membrane density perturbation reagents. Methacrylate spheres were bound to antibody molecules and used to label antigens on erythrocytes. Ghosts prepared from labeled cells were subjected to isopycnic centrifugation on continuous sucrose and dextran gradients. It was found that the labeled erythrocyte membranes had a substantially higher density than unlabeled membranes. The extent to which the membrane density was shifted on a given gradient depended on the number, size and density of the latex spheres and could be closely predicted by theory. These results suggest that the reagents and techniques described here have potential application for the isolation of plasma membranes from more complex cell types.