Immunol 1998;47:179-188 Group B streptococci (GBS) colonizing the female genital tract will often infect newborn infants during delivery. In 200 pregnant women studied, 14% were colonized with GBS in the cervix, 12% in the rectum, and 9% in both cervix and rectum. We have previously reported that antibody levels to GBS serotypes Ia, II, and III in sera and cervical secretions were increased in women colonized in the rectum and/or cervix, when analyzed by a whole-cell ELISA. Here, we report the levels of antibodies to GBS serotype III capsular polysaccharide antigen (CPS III) and to protein antigen R 4 , which are present in most GBS III strains. Compared to culture-negative women, the group of women colonized rectally had markedly elevated levels of immunoglobulin (Ig)A and IgG antibodies in cervical secretions to both CPS III and protein R 4 (P < 0.01 and P < 0.001, respectively). In sera, the corresponding differences between culture-negative and culture-positive women were less pronounced, or not present. In contrast to antibody levels to whole-cell GBS, antibody levels to CPS III and protein R 4 in cervical secretions were not significantly increased in women colonized only in the cervix, except that IgA antibodies to protein R 4 were slightly elevated (P < 0.05). These findings suggest that capsular type-specific polysaccharides and protein R 4 in a mucosal vaccine might induce protective antibodies against GBS colonization of the uterine cervix.