Fibrin deposition and exudation of plasma fibrinogen (Fg) have long been recognized as hallmarks of inflammation, cardiovascular disease and neoplasia. The Fg‐β^15–42 domain binds to the endothelial cell adhesion molecule, VE‐cadherin, promoting endothelial cell proliferation, angiogenesis and leukocyte diapedesis. Furthermore, spontaneous blood‐borne and lymphatic metastasis of some types of tumor emboli requires plasma fibrin(ogen); however, the molecular mechanisms by which this occurs are poorly understood. We sought to determine whether Fg‐β^15–42 and VE‐cadherin binding interactions promote endothelial barrier permeability and breast cancer cell transendothelial migration (TEM) using transwell insert culture systems. Synthetic peptides containing/missing residues β^15–17 critical for Fg‐β^15–42 binding to VE‐cadherin, and antibodies that bind to Fg‐β^15–21 (T2G1) and VE‐cadherin (BV9) were used to induce or inhibit Fg‐mediated permeability and TEM. Fg induced dose‐dependent permeability of human umbilical vein and microvascular endothelial but not epithelial cell barriers. Maximal Fg‐induced endothelial permeability required Fg‐β^15–42 and VE‐cadherin‐binding interactions involving Fg‐β^15–17. Fg‐induced TEM of malignant MDA‐MB‐231 and MCF‐7 breast cancer cells also required Fg‐β^15–42 and VE‐cadherin binding; however, such TEM was independent of E‐cadherin or estrogen receptor expression. In contrast, Fg did not induce TEM of nonmalignant MCF‐10A breast epithelial cells. Fg‐induced endothelial permeability was retained in the presence of MDA‐MB‐231 but inhibited in the presence of MCF‐10A cells. It is intriguing to speculate that loss of Fg‐β^15–42 binding by premalignant breast epithelial cells serves as a molecular switch to induce a highly aggressive, metastatic breast cancer phenotype. Hence, Fg‐β^15–42 represents a potential molecular target for therapeutic intervention of breast cancer metastasis. © 2009 UICC