ABSTRACT: The association between the group B beta-hemolytic streptococcus (GBBHS) and premature labor, premature rupture of the membranes, and chorioamnionitis with intact membranes is well known. However, the mechanisms involved in this association remain unclear. In this study we incubated term chorioamniotic membranes from patients undergoing elective repeat cesarean section in double-armed incubation vessels that isolated the maternal from the fetal surfaces. The GBBHS was placed adjacent to the maternal surface and incubated for 2, 4, 8, 12, and 24 hr. Control membrane specimens from the same patient were incubated under identical conditions for identical time periods but without the presence of GBBHS. Examination of the specimens by transmission electron microscopy revealed progressive nuclear and basement membrane alteration in the study group that was maximal at 12 hr. There was also a progressive decrease in desmosome counts in the study membranes. These in vitro changes support the in vivo association of GBBHS with the aforementioned obstetric complications.