The protein antigens of prototypes of five types of group B streptococcal strains were extracted with HCl or Triton X-100, separated by sodium dodecyl sulfate polyacrylamide electrophoresis, transferred to nitrocellulose, and examined by immunochemical staining. The Ibc proteins are shown to consist of at least two distinct protein antigens and their breakdown products. One antigen, the "beta" antigen, exists primarily as a 130,000 mol wt protein that is also able to bind human IgA. The "alpha" antigen, which has no known function, appears as a number of proteins of various molecular weights from 20,000 to 120,000. Another set of antigens, the R protein antigens of type III strains, has been identified as a group of acid-labile proteins varying in molecular weight from 100,000 to 130,000. In addition, two previously undescribed antigens have been found that are common to all five group B types.