The Ibc protein fraction of group B streptococci was prepared by HCL extraction of the type Ic strains A909 and 335. The fraction from strain A909 contained two protein antigens (alpha A 909 and beta A909) that could be separated by ion-exchange chromatography and isoelectric focusing. The 335 extract contained the alpha (alpha 335)-but not the beta antigen. The alpha 335 antigen was purified by similar procedures. The beta A909 antigen had a molecular weight of several hundred thousands, was immunogenic in rabbits and dissociated into several polypeptides on SDS-PAGE. Polypeptides with sub-unit molecular weights corresponding to 70,000 daltons showed antigenic activity. The alpha 335 antigen had a molecular weight of approximately 75,000 daltons as judged from gel filtration and SDS-PAGE. The antigen was immunogenic in rabbits. In contrast, the alpha A909 antigen showed neither protein lines on SDS-PAGE, nor immunogenicity in rabbits. However, the two alpha antigens showed serological crossreactivity in tests with the anti-alpha 335 serum.