Equilibrium sedimentation in caesium chloride has been used to separate and study the biologically active components of vesicular stomatitis virus (Indiana) grown in baby hamster kidney cells and chick embryos. Infective egg fluid was shown to contain two infective components of densities 1.191 g/ml. and 1.217 g/ml. A peak of complementfixing activity was found close to the 1.191 g/ml. infective band but about 50% of the activity was located between the density limits of 1.30 and 1.40 g/ml. No appreciable complement-fixing activity was found in the region of the second infective component at 1.217 g/ml. Virus suspensions from baby hamster kidney cells appeared to contain only one infective component at a density of 1.225 g/ml. In this case also, a peak of complement-fixing activity was found in close association with the peak of infectivity and between the density limits of 1.30 and 1.40 g/ml. The two infective components produced in eggs could be separated by differential ultracentrifugation and migrated through agar gels at different rates.