Summary Rubella virus centrifuged over a preformed CsCl gradient for 1.5 hr gave a sharp, opaque band at a density of 1.22. Recovery of infectivity and hemagglutinin from crude suspensions was almost complete. Electron microscopic examination of the band showed structured particles averaging 55 mµ in diameter (range was 42 to 71 mµ). Some particles showed spicule surface subunits. The number of characteristic particles in different fractions correlated well with biologic activity, offering strong evidence for the identity of rubella virus as a 55-mµ particle (average diameter). Tween 80 treatment fragmented these particles into burr-like components which were highly reactive as hemagglutinin. Its small size and the absence of 180 Å ribonucleoprotein in disrupted preparations suggests that rubella virus does not belong in the myxovirus or paramyxovirus group. Fifty-five-millimiron particles were efficient in stimulating rubella virus antibody in guinea pigs. However, neither this antibody nor post-rubella human serum agglutinated the 55-mµ particles.