A simple and reproducible method for postembedding ultrastructural immunogold labeling is described. Tissues are fixed in Carson-Millonig phosphate-buffered 4% formaldehyde, and then are embedded in LR White resin. Immunogold labeling of ultrathin sections is accomplished by means of an indirect method that employs a nonconjugated primary antibody and gold particles conjugated to antibody to immunoglobulin G. Technical aspects of fixation, embedding, and immunolabeling that influence the success of this protocol are discussed.