In vitro penetration of human chorioamniotic membranes by Campylobacter jejuni was investigated by an organ culture model. Membrane permeation was detected by an immunoperoxidase technique and viable bacterial counts of membrane homogenates. Human clinical isolates of C. jejuni inoculated on the maternal side of the membranes penetrated to the fetal side suggesting that chorioamniotic membranes constituted a weak barrier against Campylobacter infection. Chicken fecal isolates did not penetrate chorioamniotic membranes. In vitro culture conditions did not affect the viability of membranes. Human placental extracts and amniotic fluids enhanced the in vitro growth of C. jejuni. These results suggest that certain strains of C. jejuni may penetrate intact fetal membranes and this event may play a role in the pathogenesis of infection.