Group B streptococci (GBS) are the major cause of neonatal pneumonia, sepsis, and meningitis. Steps considered to be important in the pathogenesis of this infection include colonization of the rectum and vagina of the mother, aspiration of GBS into the fetal lung during or just prior to delivery, and invasion of GBS into pulmonary epithelial cells. We have previously demonstrated that GBS can invade pulmonary epithelial cells both in vivo and in vitro. Adherence of GBS to epithelial cells may play an important role in colonization of the rectum and vagina and constitute a first step in invasion of pulmonary epithelial cells. Because GBS can both adhere to and invade epithelial cells, we have developed two assays for GBS adherence which measure cell surface and not intracellular bacteria. Using these assays, we were able to demonstrate specific adherence of GBS to pulmonary epithelial cells. Adherence levels were similar at 4 and 37 degrees C and for log- and stationary-phase bacteria. Physiologic conditions vary considerably between the rectum, vagina, and lung, and a range of conditions was therefore tested. Adherence was enhanced in hypotonic solutions, while magnesium and calcium had no effect on adherence at physiologic concentrations. In comparison with adherence at neutral pH, adherence was increased 6- to 20-fold at pH 4, which is the normal vaginal pH. Neither capsular polysaccharide nor lipoteichoic acid was important for adherence in these assays. Treatment of GBS with trypsin decreased their adherence by more than 75%, indicating that surface proteins play an important role.