The vast mucosal interface separating external from internal compartments of the lung is under the surveillance of a population of blood-borne, bone marrow-derived dendritic cells (DC) characterized by constant turnover. Because these sentinel cells process foreign antigens that have penetrated the epithelial barrier and transport them to local lymph nodes, they require continuous replenishment by blood-borne cells. In the present study, the phenotype and function of DC and their precursors isolated from the vascular compartment of the lung were examined and compared with those in vena cava blood. Intravascular leukocytes were retrieved by exhaustive perfusion of the lung vasculature. Leukocytes harvested from the subdiaphragmatic vena cava of the same animal served as a source of DC in prepulmonary blood. Typical, large, major histocompatibility class II+ antigen (MHC II+) DC constituted < 1% of leukocytes from either vascular compartment. These cells expressed intercellular adhesion molecule [ICAM]-1 and lymphocyte function-associated antigen [LFA]-1 and many were ED1+ (lysosomal antigen in monocytes, macrophages, and some DC). No ED2+ cells (macrophages) were identified. Very few of the circulating DC expressed the α-like subunit of integrin recognized by the OX62 monoclonal antibody. A striking difference appeared when neutrophil-depleted leukocytes were cultured with granulocyte macrophage colony-stimulating factor (GM-CSF) for 3 d; the number of MHC II+ DC generated from pulmonary vascular leukocytes was 76% greater than that from the vena cava population. After pulse-labeling with tritiated thymidine ([3H]TdR) followed by 3 d of culture with GM-CSF, DC from either source remained virtually unlabeled, as determined by autoradiography. On the day of harvest, DC and their precursors obtained from either vascular compartment were poor stimulators of the mixed leukocyte reaction and required GM-CSF for development of their full accessory cell capability. These data suggest that, relative to leukocytes in vena cava blood, those in the lung vascular compartment are enriched in a population of mononuclear cells that are capable of differentiating into MHC II+ DC when exposed to the appropriate growth factors, including GM-CSF. This enriched population of DC precursors could represent a source from which lung DC may be readily recruited not only to replenish the normally turning-over mucosal DC, but also to participate in inflammatory reactions occurring in the lung.