The streptococcal R1 protein was studied by means of anti-R1 antibodies prepared by appropriate cross-absorption of rabbit antiserum raised against the group B streptococcal (GBS) strain ATCC 12403 (D136C), serotype III/R1. The protein was a ladder-forming antigen according to banding patterns in immunoblotting, similar to several other GBS proteins, and was susceptible to digestion by both pepsin and trypsin. Antibody-based testing revealed that 10% of Norwegian GBS isolates expressed the R1 protein, most frequently capsular antigen type V strains (72%) and less frequently type III strains (3%). None of 132 GBS strains from Zimbabwe, including 39 type V strains, expressed the R1 protein. R1-specific rabbit antibodies showed protective activity in mice challenged with a GBS type V/R1 strain. The results show that the R1 protein is an important GBS serotype marker in strains from certain geographical areas, notably for the subtyping of capsular type V strains, and that this protein is a target of protective antibodies.