Human malignant cancer tumors grafted into nude mice produce tumors containing both human cancer cells and the host's stromal cells. After short-term propagation of these tumors in vitro, the murine mesenchymal cells appear transformed and are tumorigenic in nude mice. However, established human cancer cell lines fail to similarly after adjacent murine stromal cells when used to produce tumors in nude mice. These experiments suggest that cancer cells may recruit normal cells to become malignant, qualifying the view of the clonal (unicellular) origin of cancer.