Intra-amniotic infections are believed to result from bacteria of cervical and vaginal origin which gain access to the amniotic sac. The logical sequence in this process would be bacterial attachment to the maternal surface, followed by migration through the chorioamniotic membranes to the fetal surface. Fresh sterile chorioamniotic membranes were interposed between two arms of specially constructed incubation vessels. Bacteria (Escherichia coli, group B streptococci, or Neisseria gonorrhoeae) were inoculated into the arm (containing a basal salt medium) contiguous with the maternal surface. The arm contiguous with the fetal surface of the membrane contained pseudoamniotic fluid. At intervals up to 24 hours after inoculation, the membranes were removed, washed, fixed in glutaraldehyde, and examined by means of scanning and transmission electron microscopes. The ability of group B streptococci and E. coli to attach to and invade the chorioamniotic membranes was demonstrated by this technique. It appeared that group B streptococci had a greater capacity to attach and invade than did E. coli, whereas N. gonorrhoeae predictably failed to attach.