Involvement of marrow fibroblasts in myeloproliferative disorders such as chronic myelogenous leukemia (CML) is controversial. We examined the blood leukocytes and bone marrow fibroblasts of four patients with Philadelphia chromosome- (Ph1) positive CML for evidence of rearrangement in the breakpoint cluster region (BCR). Fibroblasts were obtained by growing bone marrow in long-term culture, disposing of nonadherent cells, and passaging three times before final trypsinization and analysis. DNA from fibroblasts and peripheral leukocytes was digested with Bg1II, Bc1I, EcoRI, and HindIII restriction endonucleases and hybridized to both 3' and 5' BCR probes. Southern blot analysis of the DNA from the peripheral leukocytes in all four patients demonstrated rearrangement in the BCR. However, analysis of DNA from marrow fibroblasts showed only the normal BCR restriction fragments in all patients. This study demonstrates the absence of the Ph1 -associated molecular events in the fibroblasts of patients with CML. It is consistent with previous studies using G6PD isoenzymes to show polyclonality in CML fibroblasts and with most cytogenetic studies that did not show the Ph1 in these cells. In summary, we present further evidence that involvement of CML fibroblasts is a secondary event in the leukemogenic process.