The sedimentation constant of the infective particle in foot-and-mouth disease has been determined by modifications of the methods of Elford (1936) and Polsen (1941). A new high-speed, swinging-cup rotor was employed. A sedimentation constant of 70 Svedberg units was obtained for the infective particle in a variety of starting materials derived from guineapigs, mice and cattle. The validity of the data is discussed in relation to the accuracy with which a sedimentation constant may be determined by these methods. Ultracentrifugal studies employing inclined tubes have demonstrated that in fresh preparations the infective particle is associated with from 0 to 50% of the initial complement-fixing activity. The remaining complement-fixing activity is associated with a component of sedimentation constant 8 Svedberg units. This slower sedimenting component, if infective, contributes less than 0⋅01% of the initial infectivity. A direct and relatively precise method is described for the determination of the partition of a biological activity between two or more components of a virus system. By the use of radial and inclined tubes in non-optical procedures a correlation between these methods has been established. It is shown that the sedimentation constant of a biologically active component may be estimated by procedures based on sampling in inclined tubes. The G integral is introduced as an accurate and convenient parameter which greatly facilitates the calculation and presentation of the results of ultracentrifugal studies.