Antigen-antibody reactions are usually visualized under the electron microscope through the use of gold-conjugated secondary reagents such as protein A, or goat antiserum to rabbit immunoglobulins. This chapter considers the possibility of using the lectins in addition to antibodies for detecting carbohydrate-containing molecules in infected plant tissues. Cyto- and immunocytochemical techniques are based on the affinity properties existing between macro-molecules. Colloidal gold is a negatively charged hydrophobic sol, composed of electron-opaque, metallic particles which are capable of strong emission of secondary electrons and can absorb macromolecules under specific conditions of pH and concentration. Post-embedding techniques are the most flexible and widely used approaches in immunocytochemistry. Although there is no standard protocol for the preparation of tissue for on-grid immunocytochemistry, the main objective remains the preservation of both the ultrastructure and antigenic immunoreactivity. The immunocytolocalization of plant proteins has been an exciting area that has provided new insights into the complex mechanisms of disease resistance.