To identify women colonized with group B streptococci during parturition, we used pooled type-specific fluorescent antibody to examine vaginal swabs enriched by preincubation in selective broth medium. In preliminary experiments, group B streptococcus strain III-Bell was reliably detectable with fluorescent antibody at concentrations of greater than 10(5) colony-forming units per ml, achieved after 6 h of incubation of small inocula (18 to 26 colony-forming units). Of the vaginal swabs from 924 parturient women examined prospectively by both fluorescent antibody and selective bacteriology techniques, group B streptococci were isolated in 154. The sensitivity of the fluorescent antibody technique increased with increasing incubation time and ranged from 49% (3 to 6 h) to 81% (7 to 12 h) to 83% (13 to 18 h) to 93% (greater than 18 h). Colonized mothers identified within 6 h by the fluorescent antibody technique had higher rates of vertical transmission to their newborn infants (61%) than colonized mothers whose fluorescent antibody examinations were negative within this time interval (32%; P = 0.027). However, because of the timing of their admissions, none of the colonized mothers of the four infants who developed early-onset group B streptococcal sepsis were identified with fluorescent antibody until after delivery. Although its sensitivity approaches selective culture methods after 6 h of incubation, fluorescent antibody examination of vaginal swabs does not appear to offer a practical approach to identifying colonized parturient women for intrapartum antibiotic prophylaxis of group B streptococcal infection.