Establishing the presence of various cell types in long-term marrow culture (LTMC) has an important bearing on understanding the regulation of stromal cell-related hemopoiesis. Controversy has surrounded the identity of the very large cells in murine LTMC; they have been given a variety of designations, including blanket cells. Using dual immunogold labeling with a recently derived monoclonal antibody, H513E3, and anti-human factor VIII antibodies, we have conclusively located endothelial cells in LTMC. Endothelial cells are relatively large, with thinly spread cytoplasm, and they overlie macrophages, granulocytes, and other less differentiated developing hemopoietic cells. Previously, demonstration of endothelial cells in LTMC had been difficult due to lack of specific markers in the mouse. We have demonstrated that LTMC endothelial cells have the exact location and ultrastructural characteristics as the previously described blanket cells. We propose that previous designations should not be continued and that these cells should be referred to as endothelial cells. The known functions of endothelial cells now become relevant to the understanding of stromal regulation of hemopoiesis.