Mesenchymal stromal cells (MSCs) have enormous potential for the regeneration of bone, cartilage, and other tissues derived from primitive mesoderm. Despite extensive research, there is still no single marker that reliably identifies MSCs within the bone marrow. Using immunocytochemistry and flow cytometry, we demonstrate here that the neural ganglioside GD2 is expressed by MSCs either newly isolated from bone marrow or expanded in tissue culture; this finding was supported by reverse transcriptase–polymerase chain reaction (RT-PCR) analysis showing expression of the mRNA for GD2 synthase, an essential enzyme for GD2 biosynthesis. GD2 was also expressed on MSCs isolated from adipose tissue, but not on foreskin fibroblasts. Importantly, MSCs were the only cells within normal marrow that expressed this marker. Thus, GD2 appears to be the first reported single surface marker that uniquely distinguishes MSCs from other marrow elements. GD2 may prove valuable to study MSC biology and for the preparation of MSCs for clinical applications.