We report the biological characteristics of leukaemic blasts from two cases of acute leukaemia with an interstitial deletion of the long arm of chromosome 9 (9q-). Case 1 (FAB: M1) showed del(9)(q12q22) as the sole karyotypic anomaly, and case 2 (FAB: M1) presented del(9) (q12q22) in association with trisomy 10. In both cases, leukaemic blasts presented unique cytological features, such as prominent vacuoles on Giemsa staining, or strong localization of myeloperoxidase resembling 'pseudo-Chediak-Higashi' granules. Immunophenotyping of blasts revealed the biphenotypic expression of T-lymphoid/myeloid antigens (CD2, CD7/CD33) in addition to CD34. Neither T-cell receptor beta (TCRB), T-cell receptor gamma (TCRG) nor Ig heavy chain (IGH) genes were clonally rearranged. Furthermore, there was neither rearrangement nor expression of ABL, which is located at 9q34, indicating that the deletion involved bands centrometric to 9q34 did not induce the activation of ABL. DNA synthesis of the blasts was stimulated (stimulation index greater than 2.0) in the presence of interleukin (IL)-3, IL-4, granulocyte colony-stimulating factor or erythropoietin (Epo). IL-3 and IL-4 could also support the in vitro growth of leukaemic blast colonies, and the IL-3- or IL-4-dependent blast colony growth was synergistically enhanced by the addition of IL-6 or Epo. These observations imply that T-lymphoid/myeloid or pluripotent stem cells may be closely involved in the development of 9q- AML.