The effects of two recombinant human CSFs (G-CSF and GM-CSF) on the growth of blast progenitors from 36 acute myeloblastic leukemia patients were studied in methylcellulose and suspension cultures. Blast colony formation in methylcellulose and the growth of blast progenitors in suspension were stimulated by G-CSF or GM-CSF. Their responses to CSFs were different from those of normal myeloid progenitors. First, the sensitivity of blasts to 0.01 ng/ml of G-CSF and 0.001 ng/ml of GM-CSF was significantly increased compared with normal. Second, in more than 70% of patients, the pattern of the responsiveness to the two CSFs was aberrant compared with ordered response in normal subjects. Third, in about half of the patients, combination of G-CSF and GM-CSF showed synergism for the growth of blast progenitors in both culture methods, whereas negligible or no synergism was observed in normal subjects. Finally, when stimulated by G-CSF, GM-CSF, or both, a significant relationship was noted between blast colony formation in methylcellulose and blast progenitor growth in suspension, suggesting that CSFs do not affect the balance between self-renewal and terminal divisions of blast stem cells.