Group B streptococci (GBS), mainly serotype III, are the major cause of neonatal pneumonia, sepsis and meningitis. Virulence potential of GBS strains may determine the outcome of host colonization or infection. Because the lung constitutes a first step in GBS systemic invasion processes, we investigated the adherence and invasion mechanisms of GBS-III clinical isolates to non-polarized human bronchial epithelial 16 HBE 14o- cell line. The presence of genotypic 162-kb and 183-kb virulence markers in all strains was also examined by PFGE. The 162-kb fragment was detected in both liquor (GBS-III 90356) and vagina (GBS-III 39A) isolates, while 183-kb fragment was only observed in strains (GBS-III 39A, 89A, and 80340) isolated from vagina of asymptomatic carriers. The actin-dependent ability to internalize within non-polarized epithelial respiratory cells was demonstrated only by GBS-III clinical isolates presenting the 162-kb virulence marker. GBS-III 39A strain isolated from vagina exhibiting both 183-kb and 162-kb fragments showed a more efficient adherence and invasion properties than GBS-III 90356 isolated from liquor (P<0.001). Our data suggest the expression of additional bacterial virulence factors that may favor adherence and survival to non-polarized respiratory epithelial cells with consequent development of systemic diseases by GBS-III, including some strains isolated from asymptomatic carriers.