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 relationship [Formula: see text] was 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.