Over 1000 monoclonal antibodies (MAB), previously characterized in terms of reactivity with various haemopoietic cells and including representatives from each of the currently known Clusters of Differentiation (CD), have now been evaluated by us for reactivity against human endothelial cells. In the current study, screening was performed on both fixed, and on non-fixed, endothelial cells derived and cultured from umbilical cord veins. Results indicate that endothelial cells co-express a wide variety of antigens that are otherwise characterized by expression on haemopoietic cells. The most intense and consistent degree of reactivity was seen using MAB to CD-9, CD-13, CD-29, CD-31, CD-44, CD-46, CD-51, CD-54 and CD-55, although reactivity was also observed using MAB belonging to CD-41, CD-47, CD-49, CD-61 and CD-71, in addition to a number of non-clustered MAB.