We have previously isolated mouse embryonic cell lines with endothelial potential using a simple empirical approach. In an attempt to isolate similar cell lines from adult mouse bone marrow (BM), BM cells were cultured on mitotically inactive mouse embryonic fibroblast (MEF) feeder cells. Several cell lines with putative endothelial potential were generated. They expressed endothelial-specific genes and formed vascular-like structures when plated on matrigel. When transplanted into appropriate mouse models, they incorporated into the endothelium of the vasculature. Similar cell lines were also obtained using human or porcine BM. None of these lines induced tumor formation when transplanted into immunodeficient Rag1-/- mice. However, all the lines were aneuploid with genetic markers from BM samples and the MEF feeder, suggesting that they resulted from a non-species-specific fusion of a BM cell and mitotically inactive MEF. Together, these lines demonstrated for the first time that BM cells can also undergo fusion with commonly used mitotically inactive feeder cells. Although these fusion cell lines were culture artifacts, their derivation would be useful in understanding fusion of BM cells with other cell types, and their endothelial potential will also be useful in characterizing endothelial differentiation.