This report describes a monoclonal antibody (MAb) generated by immunization of mice with cell suspensions of capillary-rich fragments of mammary carcinomas. The antibody (EN 7/44) belongs to the IgM class and detects a 30.5 kDa antigen which is found in the cytoplasm of human placental and umbilical vein-endothelial cells (HUVEC) and on the surface of tumor endothelium. Using indirect immunofluorescence and immunoperoxidase techniques, we did not find the MAb in peripheral blood cells or in any human cell lines tested, nor in endothelial cells from normal, nonproliferating adult tissues. Positively staining endothelium was found in the placenta, the umbilical vein and in proliferating normal tissues (intestine). Positive endothelial cells were found in acute inflammatory reactions and in tumors. In the latter, the strongest reactions were seen in newly formed budding capillaries, which were identified by their reactivity with Ulex europaeus I-lectin and antibodies against F VIII-RAG. In tumor tissues, large vessels could be positively stained with the EN 7/44 antigen, in contrast to inflammatory tissues. It is concluded that endothelial cells at the tip of a budding capillary express distinct phenotypic characteristics.