Mesenchymal stromal cells (MSC) are multipotent cells that can be isolated from a number of human tissues. In cancer, MSC have been implicated with tumor growth, invasion, metastasis, drug resistance and were even suggested as possible tumor-initiating cells in osteosarcoma (OS). However, MSC from OS and their possible tumor origin have not yet been thoroughly investigated. Therefore, primary OS mesenchymal progenitors and OS-derived MSC were studied. OS samples contained very high frequencies of mesenchymal progenitor cells as measured by the colony-forming unit fibroblast (CFU-F) assay (median: 1,117 colonies per 10(5) cells, range: 133-3,000, n = 6). This is considerably higher compared to other human tissues such as normal bone marrow (BM) (1.3 ± 0.2 colonies per 10(5) cells, n = 8). OS-derived MSC (OS-MSC) showed normal MSC morphology and expressed the typical MSC surface marker profile (CD105/CD73/CD90/CD44/HLA-classI/CD166 positive, CD45/CD34/CD14/CD19/HLA-DR/CD31 negative). Furthermore, all OS-MSC samples could be differentiated into the osteogenic lineage, and all but one sample into adipocytes and chondrocytes. Genetic analysis of OS-MSC as well as OS-derived spheres showed no tumor-related chromosomal aberrations. OS-MSC expression of markers related to tumor-associated fibroblasts (fibroblast surface protein, alpha-smooth muscle actin, vimentin) was comparable to BM-MSC and OS-MSC growth was considerably affected by tyrosine kinase inhibitors. Taken together, our results demonstrate that normal, non-malignant mesenchymal stroma cells are isolated from OS when MSC culture techniques are applied. OS-MSC represent a major constituent of the tumor microenvironment, and they share many properties with BM-derived MSC.