Electrophoretic studies were made on vaccine virus, collodion particles, and glass particles suspended in 0.01 molar buffer solutions at pH 7.9, in which the moving boundary method was used. In some experiments, uncoated particles were used; in others, particles were coated with proteins and then resuspended in the buffer solution after a washing; in still others, an excess of protein which had been used to coat the particles was included in the buffer medium. Streaming boundaries were obtained with all dilute suspensions of particles in solutions containing no soluble protein instead of the flat ones usually observed with the Tiselius moving boundary technique. This boundary artifact was suppressed by maintaining a density gradient of sufficient magnitude in association with the moving boundary to counteract the tendency of endosmotic flow. This was done partially by increasing the concentration of the particles in the suspensions, and almost completely by retaining an excess of soluble-coating substance in the solutions containing the particles. The mobility of elementary bodies of vaccinia corresponds to that found for the heat-stable (S) antigen. This value was not altered by drying, heating, ether extraction, or simple washing, but was materially increased by treatment with the surface active detergent (duponol) which presumably altered the nature of the surface of the virus particles. Collodion particles coated with the heat-stable antigen of vaccinia had the same mobility as elementary bodies under comparable conditions. Glass particles coated with normal rabbit serum moved at the rate of albumin, the fastest serum component in the buffer solutions used. However, both collodion particles and vaccine virus moved at a somewhat slower rate when they were similarly coated and measured in the presence of an excess of serum in the solutions. This was probably due to adsorption of a small amount of one of the slower components (globulin) of rabbit serum on the surface of the particles. Simple washing after treatment seemed to remove the coating of serum proteins, at least in part, from both collodion particles and elementary bodies of vaccinia.