In the course of prospective epidemiological studies of group B streptococcal (GBS) colonization and infection, we surveyed 401 paired maternal and cord sera (obtained at delivery) and 23 sera from patients with systemic type II infection for IgG antibody to GBS type II. Type II carriers were more likely to have antibody (greater than 2 micrograms/ml) than were those carrying other GBS types, whereas noncolonized patients were the least likely to have antibody. The overall prevalence of levels of antibody greater than 2 micrograms/ml was estimated to be approximately 6%, on the basis of assay results of the 401 maternal-cord pairs and adjusted for known-colonization status for the entire population of 8,928 deliveries that occurred during the study period. The majority of patients with infection had antibody levels less than 2 micrograms/ml. Five patients, however, had antibody present at levels ranging from 2.7 to 5.8 micrograms/ml. These findings suggest that "antibody deficiency" was widespread and was not by itself a useful determinant of risk for disease caused by GBS type II.