The cytogenetic status of bone marrow stromal elements obtained from six patients with Ph1-positive chronic myelogenous leukemia (CML), two in blast crisis, was studied in vitro utilizing the potential of marrow to form surface-adherent colonies morphologically compatible with mesenchymal elements. We demonstrated the absence of both the marker chromosome and other chromosomal abnormalities in all the fibroblastic colonies studied, indicating that the progenitors of such colonies (plaque-forming units in culture, PFU-C) are not closely related to hematopoietic elements including macrophages. This supports previous reports suggesting that the stromal elements in myelofibrosis associated with CML are not derived from the primary Ph1-positive malignant clone but represent a stromal reactive component of benign or independent malignant potential.