Three hemagglutinins were separated by equilibrium density centrifugation in CsCl of Sindbis concentrates obtained from infected chick-embryo cultures. They were designated heavy fraction (HF, density 1.240 g/ml), intermediate fraction (IF, density 1.212 g/ml), and light fraction (LF, density 1.191 g/ml). On the basis of work published by other authors, HF is considered to consist of Sindbis virions and LF to consist of non-infectious coreless particles and other membranous structures. The third hemagglutinin fraction, IF, appeared in the culture fluid at about the same time as the virions (HF) and before the non-infectious LF. In a series of rebanding experiments in CsCl gradients the relationshipwas demonstrated. The constant buoyant density of IF and results of the reconstitution experiments described suggest that IF consists of virions surrounded by LF material in a fixed and constant proportion. It is not established whether IF plays any role in the intracellular replication of the virus.