Antiserum to each of the alpha and beta antigens of the Ibc protein fraction of group B streptococci was raised in rabbits. A possible protective effect of the antisera in mice challenged by group B streptococci was evaluated. Antiserum against each of the antigens significantly increased the survival rates of mice challenged by bacteria which produced the corresponding antigen, but not of those challenged by bacteria which produced only the heterologous antigen. Animals challenged with a strain which produced both of the alpha and beta antigens were protected by antiserum to the beta antigen but not by antiserum to the alpha antigen. In this case the effect of the anti-alpha serum was a postponement of death of the animals. The antisera showed no protection of animals challenged by a group B streptococcal strain which did not produce any of the Ibc protein antigens.