RRSV sedimented as 3 components, T, M and B, with sedimentation coefficients of 52, 92 and 130 S, respectively, and containing 0, 30 and 44% RNA but serologically indistinguishable. M and B were homogeneous in CsCl gradients with buoyant densities of 1.43 and 1.52 g/cm3. Infectivity was associated with the B component. The RNA was single-stranded and consisted of 2 predominant species, one indistinguishable from RNA-2 of tobacco ringspot virus and the other migrating more slowly than RNA-1 of TRSV. The M particles contained 1 molecule of RNA-2; B particles either 1 molecule of RNA-1 or, probably, 2 of RNA-2. The B particles of TRSV formed 2 buoyant density classes in CsCl but those of RRSV were homogeneous.