During a 19-month period selective vaginal Gram stain and culturing were performed prospectively in all women admitted to the delivery room at risk for low birth weight delivery secondary to preterm premature rupture of the membranes or for prolonged membrane rupture secondary to term premature rupture of the membranes. Gram stain was performed and read immediately on admission. Cultures were also obtained simultaneously and sent for microbiologic evaluation. All women with Gram stains positive for gram-positive cocci were treated with intravenous ampicillin in the event of labor, unless vaginal culture results were already known and were negative for group B beta-hemolytic streptococcus. A total of 72 women had samples taken, and 30 (41.7%) were positive for gram-positive cocci. Nine (30%) of these were subsequently positive for group B beta-hemolytic streptococci by culture. There were no cultures positive for group B beta-hemolytic streptococci in the Gram stain-negative group. Gram stain sensitivity was 100% for predicting the presence of group B beta-hemolytic streptococci in the study population, and specificity was 66.7%. Selective vaginal Gram stain provides an effective and rapid screen for identifying the presence of group B beta-hemolytic streptococci and allows for immediate institution of appropriate antibiotic therapy in the event of onset of labor before the availability of culture results.