A MoAb RMAC8 was generated by immunizing Balb/c mice with cultured human umbilical vein endothelial cells (HUVE). The molecule recognized had a mol. wt of 180 kD non-reduced, 95 kD after reduction and 66 kD in its reduced and N-deglycosylated form. Sequential immunoprecipitation studies with the MoAb 44G4, which recognizes the O- and N-glycosylated homodimer endoglin, showed that both MoAbs recognize the same molecule on HUVE and phorbol myristate acetate (PMA)-stimulated U937 cells. The distribution of the RMAC8-recognized molecule was the same as that described for endoglin, i.e. arterial and venous endothelium, myelomonocytic and pre-B leukaemia cells and cell lines; however, unlike 44G4, RMAC8 also reacted weakly with monocytes and strongly with in vitro differentiated macrophages as well as peritoneal and alveolar macrophages. This distribution of endoglin was confirmed by Northern blot analysis using a full length endoglin cDNA probe. These studies suggest that endoglin is a differentiation marker on macrophages.