Antiserum to each of the α and β 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 α and β antigens were protected by antiserum to the β antigen but not by antiserum to the α antigen. In this case the effect of the anti‐α 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.