Bacterial ascending from the cervix may infect the membranes and amniotic fluid, contributing to their premature rupture. In the present in vitro study, isolated amniotic membranes were exposed to different concentrations of disintegrated bacterial cells. Collagenolysis was estimated by the ratio of acid-soluble/insoluble hydroxyproline. The amniotic membrane proteins were separated by electrophoresis after incubation with 14C proline and disintegrated bacterial cells. The proportion of soluble hydroxyproline increased with the bacterial concentration used, and the radioactivity decreased in proteins of the 30- to 80-kD range. The data support the notion that bacteria contain collagenolytic enzymes affecting human amniotic membranes.