INTRODUCTIONIn post-embedding methods of immunogold staining, the cells or tissues are fixed chemically or cryoimmobilized, dehydrated, and embedded in epoxy or acrylic resins. Thin sections (50-70 nm in thickness) are cut using an ultramicrotome with a diamond knife, using a water bath to collect the sections as they slide off the knife. The sections are stretched with solvent vapor or a heat source and collected onto either bare or plastic-coated nickel grids. The sections are then stained immunochemically with primary antibodies raised against antigens exposed on the surface of the sections. The primary antibodies are visualized by staining immunochemically with secondary antibodies raised against the species and isotype of the primary antibodies, conjugated to colloidal gold particles. The immunochemically stained sections are then contrast stained with salts of uranium (uranyl acetate) and lead (lead citrate) to reveal the ultrastructure of the cells, and are finally viewed by transmission electron microscopy (TEM). LR White was introduced as a low-toxicity alternative to epoxy resins, which frequently contained carcinogens. Unlike the simplest acrylic resins, in which monomers are polymerized to form long chains, the LR resins contain aromatic cross-linkers to improve the stability of the sections under the electron beam. LR White and Gold both have very low viscosity and readily penetrate, even into dense tissue. In this protocol, aldehyde-fixed tissue is dehydrated in ethanol, impregnated in LR White resin and polymerized under vacuum or in a nitrogen atmosphere before sectioning and immunogold staining.