We investigated the marrows of 19 patients with advanced Philadelphia chromosome positive (Ph+) chronic myeloid leukemia (CML) in long-term marrow culture (LTMC) to determine the frequency of loss of clonogenic leukemic cells in vitro. Sixteen patients were in first chronic phase at a median of 24 months from diagnosis and had received prior therapy with busulphan and/or hydroxyurea. The effect of interferon therapy on loss of Ph+ clonogenic cells in LTMC was also investigated. Of 16 patients who had not previously received interferon, complete loss of Ph+ progenitors was documented by 4-5 weeks in the LTMCs from two (12.5%). Ph+ progenitors persisted at 4-5 weeks in the LTMC derived from 12 patients. Marrows from nine patients treated with interferon were also established in LTMC. Cultures from four patients did not yield colonies with detectable metaphases at 3-5 weeks, while Ph+ clones were present in the cultures initiated with marrows from five patients. Mean hematopoietic colony yields from the adherent layer at 2-4 weeks, and from the supernatant layer at 1-3 weeks, of cultures derived from interferon-treated patients were significantly lower than in LTMCs of patients not treated with interferon (p less than 0.05). The results indicate that in previously treated patients with late chronic phase CML there is a low frequency of conversion of Ph-negative hematopoiesis in long-term culture. Interferon therapy is associated with impaired progenitor yields in LTMC and does not improve selective loss of Ph+ progenitors.