A new murine monoclonal antibody (MAb), E-9, has been raised using tissue-cultured human umbilical vein endothelial cells. The antigen recognized by this MAb is a peptide of 170 kDa under non-reducing conditions and 96 kDa under reducing conditions. MAb E-9 showed marked heterogeneity in its distribution in various tissues. The antigen recognized by it was present in vascular endothelial cells of all tumours, foetal organs and in regenerating and inflamed tissues. It stained a few normal tissues. However, with the exception of tonsils, staining tended to be weak and limited to a few blood vessels, as revealed by double staining using pan-endothelial antibody (CD31) and antibody to von Willebrand factor, another marker of vascular endothelium. Surprisingly, blood vessels within the placental villi were completely negative. The function of the antigen recognized by MAb E-9 is not known, but its evaluation and use should increase our understanding of angiogenesis.