The c protein antigen of group B streptococci has been reported to consist of at least two acid extractable antigens, c alpha and c beta. We previously reported that strain FZ301 contain the c alpha and novel protein antigen, named as S antigen by Denka Seiken, and the c alpha antigen of this strain exhibit a more complicated structure. In this report described the analysis of the diversity of the c alpha antigen by immunoelectrophoresis and immunodiffusion. Hot acid extract from strain A909 produced a single precipitin band against the anti-121424c alpha serum but the 121424 extract produced double precipitin bands by immunoelectrophoresis. These c alpha antigens showed anodic migration, but the 121424c alpha antigens tended to migrate slower than the A909c alpha antigen. Moreover, the anti-A909c alpha serum produced a single precipitin band when tested after electrophoresis of the 121424 extract. When examined by immunodiffusion, the anti-121424c alpha serum produced two distinct precipitin bands with the 121424 and FZ301 extract, and these were reactions of identity. However, the A909 extract produced only one band which joined one of the two bands produced with the 121424 extract. These results shown that the strain 121424 contain c alpha and S antigen as well as strain FZ301. The precipitin band of the c alpha antigen in the A909 extract against the anti-A909 serum formed the spur with the reaction of the c alpha antigen in the 121424 or FZ301 extract on immunodiffusion. Moreover, the c alpha antibody in the anti-A909 serum failed to removed completely by repeated adsorption with strain 121424.(ABSTRACT TRUNCATED AT 250 WORDS)