In order to take full advantage of recent developments in the electron microscopic examination of cellular ultrastructure and composition, it is necessary to develop specific electron stains capable of identifying and localizing a wide variety of macromolecular components of cells. To this end, antibody conjugates have been prepared by chemically coupling the highly electron-scattering ferritin molecule to antibody. Antigen-antibody precipitations with these ferritin-antibody conjugates have demonstrated that under the appropriate conditions they retain the specific binding properties of the antibody from which they are prepared. An electron microscopic study has been made of aggregates of tobacco mosaic virus and its ferritin-conjugated antibody. The aggregates were prepared in solution and then sprayed onto specimen screens. The electron micrographs reveal that the conjugate specifically attached to, and delineated, the virus rods. The chemistry, structure, and resolving power of the ferritin-antibody conjugates, the specificity of their reactions with homologous antigen, and the nature of the problems to be faced in application of these conjugates to the study of the internal antigens of cells are discussed.