There are three major surface-localized protein antigens of group B streptococci: c, R, and X. Their precise role in human immunity to group B streptococci has not been defined. Studies of the c protein suggested that type II strains possessing both trypsin-resistant and trypsin-sensitive components of the c protein were less easily killed in vitro and were more virulent in an infant rat model of infection as compared with type II strains that do not bear these proteins. The c protein components were immunogenic in mice and rabbits. Polyclonal rabbit antisera were protective in the infant rat model of bacteremia/sepsis and facilitated killing of type II strains bearing the c protein in an in vitro opsonophagocytic bacterial killing assay. The role of the IgA-binding capacity of the c protein in altering the interaction of group B streptococcal strains with host defenses remains undefined at this time.