Clonogenic capacity of bone marrow progenitors and stromal layers established from bone marrow of 12 patients with CML and 13 healthy controls were evaluated. The initial BFU-E and CFU-GM contents were slightly higher in the CML patients (p > 0.05) in contrast to CFU-GEMM. CFU-GEMM was lower in the patients compared to healthy controls (p < 0.001). In long-term cultures, the number of non-adherent cell population and total clonogenic progenitor cell content decreased gradually in both groups. Weekly evaluation of stromal confluency of adherent cells revealed that establishment of adherent stromal layer was slower in CML patients than in control samples (p < 0.05). At the end of fourth week, the number of samples presenting confluency was 41.7% in the CML group compared with 92.3% in the controls. The initial CD34 positive cell content of the bone marrow samples was similar in both groups. Although CD34 positive cell number in the adherent stromal layer was well preserved in the control group at the end of 4 weeks, this figure decreased significantly in the CML group. The numbers of total adherent cells as well as the total clonogenic progenitor content of adherent layer were also lower in the CML group (3.03% vs 98.2%). When normal CD34+ cells were cultured on IFN-alpha-treated stromal layer followed by the assessment of the long-term culture initiating cells, a reduced capacity to support hemopoietic growth was observed with IFN-alpha-treated normal stroma. This reduction was even higher when CML stroma was treated with IFN-a followed by the seeding of the normal CD34+ cells on this stromal layer (26.9% vs 42.8%). These findings show that stromal cells are abnormal in CML patients as well as the progenitor cells, and IFN-alpha treatment causes further defects of the stromal cells. (C) 1999 Wiley-Liss, Inc.