The molecular constituents of the paracellular clefts in human placental microvessels were investigated using antibodies against PECAM-1, pan-cadherin, A-CAM (N-cadherin), cadherin-5 and two types of integrins (those recognised by antibodies to the beta 1 chain and alpha v beta 3). Ultrastructural localisation of these molecules in ultrathin frozen sections of human term placentae was attempted using colloidal gold immunocytochemistry, after establishing their presence by indirect immunofluorescence. At the light microscopical level, the endothelial paracellular clefts were found to be immunoreactive to the antibodies against PECAM-1, cadherin-5 and pan-cadherin, but not the integrins. The latter showed diffuse distribution in the endothelium and in the abluminal interstitial space. PECAM-1 and pan-cadherin were also seen in the cytoplasm and luminal surface of the endothelium. Immunoelectron studies revealed that the cadherins and PECAM-1 were present in the wide regions of the paracellular clefts, but not in tight junctional regions. Using immunocytochemistry, these wide junctional areas were found to be associated with the cytoskeletal linking molecules vinculin and alpha-actinin. These regions may therefore contain adherens-type junctions. Cadherin-5, localised by two different monoclonal antibodies, 7B4 and TEA, was the only antigen which was cleft-specific, the others also being seen in the cytoplasm of the microvascular endothelium. Cadherin-5 and pan-cadherin were co-localised in the same wide junction, but were usually seen to occupy different microdomains of, and different wide zones of, the same cleft. The cell adhesion molecules localised in the paracellular wide junctions of the human placental microvessels may play a role in maintaining the intercellular spacing between endothelial cells, and may be part of a paracellular "fibre matrix" with permeability-restricting properties.