PECAM-1 is a surface membrane glycoprotein of 130,000 daltons found on platelets, endothelial cells, monocytes, neutrophils, and certain T-cell subsets. Except for bone marrow precursor cells, it does not seem to be found outside the vasculature. Previous studies have shown that PECAM-1 is a member of the immunoglobulin gene superfamily that localizes to the intercellular junction in cells, like endothelial cells, that grow in monolayers. We and other laboratories have been involved in studies designed to define the role of PECAM-1 in various cell types. Mammalian expression vectors have been constructed that express all, or selected domains, of the PECAM-1 protein. These vectors have been used to induce PECAM-1 expression in a variety of both monolayer and suspension cell types. Domain-specific anti-PECAM-1 monoclonal antibodies, synthetic peptides, and heparin were also used to define structurally and functionally important domains of this membrane glycoprotein. Protein synthesis, subcellular localization, and association with the cytoskeletons of various cells expressing PECAM-1 were examined. From these studies we have obtained evidence that: (1) PECAM-1 redistributes to the border of monolayer cells when they contact each other. (2) Border localization may result from homophilic interactions, i.e. PECAM-1 on one cell binds PECAM-1 on the adjacent cell. (3) Suspension cells such as L-cells or neutrophils may also interact via PECAM-1 mediated interactions, but an as yet unidentified ligand probably participates in a heterophilic, rather than homophilic, type of interaction. (4) Intracellular movement of PECAM-1 in both monolayer and suspension cell types requires active participation of the cytoskeleton.(ABSTRACT TRUNCATED AT 250 WORDS)