Abstract. Polymorphonuclear leukocytes (PMN) traverse an endothelial cell (EC) barrier by crawling between neighboring EC. Whether EC regulate the integrity of their intercellular adhesive and junctional contacts in response to chemotaxing PMN is unresolved. EC respond to the binding of soluble mediators such as histamine by increasing their cytosolic free calcium concentration ([Ca++] This process can be initiated by PMN in response to soluble chemoattractants or by endothelial cells (EC) in response to cytokines such as interleukin 1 OL-1) and tumor necrosis factor (8, 29). Cell surface molecules including the CD11/CD18 complex of proteins on the PMN surface (12) and intercellu-