The physical and immunologic relationships between infectious rubella virus, the hemagglutinin (HA) and the two complement-fixing (CF) antigens were investigated. The HA eluted from Sephadex G 200 together with the infectious virus and the large-particle CF antigen, and also equilibrated with these two antigens in sucrose gradients at densities of 1.21 to 1.19 g/ml. No HA activity was associated with the small-particle CF antigen. Both HA and infectivity were absorbed into chick erythrocytes, suggesting that the HA is a part of the infectious moiety, but a variable proportion of the large-particle CF antigen was not absorbed onto erythrocytes. Antisera produced by immunization of rabbits with large- or small-particle antigens showed reciprocal crossing in CF tests. Immunization of rabbits with the noninfectious small-particle antigen elicited the formation of neutralizing and HI antibodies as well as CF antibodies. Large- and small-particle CF antigens were also indistinguishable in tests with human sera from natural rubella infections.