Vesicular stomatitis virus grown in baby hamster kidney cells was fractionated by centrifugation in a linear sucrose gradient. Three well separated peaks of complement fixing activity were obtained. The major part of the infectivity was associated with the most rapidly sedimenting complement fixing fraction. All three fractions possessed immunogenic properties in guinea pigs after inactivation with 0.05% acetylethyleneimine. The intermediate peak was the most active. Electron microscopy has revealed that each of the three fractions obtained by sucrose gradient centrifugation has a distinctive appearance. Peak A consists of complete particles and partially disintegrated particles of virus. Peak B is overwhelmingly composed of rosettes and helices together with a certain number of particle fragments. Peak C contains the smaller size range of rosettes and a considerable amount of cell material. The relationship of the appearance of the three fractions in the electron microscope to their immunizing properties is discussed.