The endothelial contribution to vascular disorders has been widely documented in experimental models. However, its implication in human pathology is difficult to investigate, owing to the paucity of noninvasive methods and of specific endothelial markers. The enumeration of circulating endothelial cells (CEC) released in peripheral blood after vascular injury represents a direct exploration of the endothelium. For this purpose, we have produced a monoclonal antibody (S-Endo 1), which recognizes CD 146, a molecule expressed on all types of human endothelial cells but absent from haemopoietic cells. Using this antibody, we have designed a specific and sensitive immunocapture test, which allowed us to detect high numbers of CEC in thrombotic, infectious or immunological disorders, while CEC were found to be very rare ( < 3/ml) in normal subjects. This quantitative approach using CEC might prove useful as a marker of vascular wall injury. Their enumeration is of interest in the clinical follow-up of vascular disorders, in the evaluation of therapeutic effectiveness or in the direct diagnosis of infectious diseases involving intra-endothelial microbial agents. Furthermore, an immunological and/or functional study of CEC could allow one to assess their procoagulant and proadhesive properties, as well as their viability, opening new perspectives for CEC investigation in vascular pathology.