The purpose of this study was to determine the reliability of the ICON immunoassay in detecting vaginal group B Streptococcus in pregnant women. Vaginal cultures were obtained at the time of sterile speculum examinations from 300 laboring women. The frequency of vaginal group B streptococcal colonization was 10%. A duplicate vaginal swab was used in the immunoassay, which required less than 10 minutes for completion. The immunoassay could reproducibly detect 3+ to 4+ growth (greater than 10(5) colony-forming units per milliliter of vaginal fluid), which was considered heavy colonization. The overall sensitivity, specificity, positive predictive value, and negative predictive value were 33, 95, 43, and 93%, respectively, compared with culture. For heavy colonization (3+ to 4+ growth), the sensitivity, specificity, positive predictive value, and negative predictive value were 100, 95, 32, and 100%, respectively. The immunoassay is a highly sensitive and specific test for detection of heavy vaginal colonization by group B streptococci in obstetric patients, but cultures are required to detect light group B streptococcal colonization.