Rationale: The role of monocytes in acute endotoxemia has been ascribed to systemic release of mediators within the central circulation. Little is known about the potential role of 'marginated' monocytes in regulating microvascular inflammatory signaling. Objectives: To investigate whether lung-marginated monocytes can locally activate pulmonary endothelial cells through cell contactdependent interactions in early endotoxemia. Methods: Mice were challenged with lipopolysaccharide to produce acute endotoxemia and pulmonary vascular injury. Adoptive transfer of ex vivo lipopolysaccharide-stimulated donor leukocytes to recipient mice was also performed to evaluate cell-associated inflammatory signaling between monocytes and endothelial cells within the lung. Cell suspensions from excised lungs were analyzed by flow cytometry for expression of tumor necrosis factor- (TNF) on monocytes and cell adhesion molecules on endothelial cells. Results: Substantial numbers of monocytes rapidly marginated to the lungs after endotoxin challenge in mice, and lung-marginated monocytes expressed significantly higher levels of membrane TNF than circulating monocytes, due to higher TNF production by the marginated cells. Injection of activated wild-type donor leukocytes to wild-type or TNF receptor double knock-out recipients demonstrated that lung-marginated monocytes can induce TNF-dependent upregulation of adhesion molecules on pulmonary endothelial cells. Injection of activated donor leukocytes from TNF knock-in mice that express uncleavable mutant membrane TNF, also induced adhesion molecule upregulation in wild-type recipients without a systemic soluble TNF release. Conclusions: These results reveal a previously unacknowledged role for lung-marginated monocytes in early endotoxemia, exerting local, cell-associated TNF signaling within the pulmonary microcirculation, contributing to evolution of pulmonary vascular injury.