Bone marrow cells from 99 patients with acute myeloid leukemia were cloned in either agar stimulated by leukocyte feeder layers (AG/F) or methylcellulose supplemented with medium conditioned by phytohemagglutinin stimulation of leukocytes (MC/P). Although cell growth in the two systems was correlated (r = 0.74, p less than 0.0001), there was increased formation and size of clusters and colonies in AG/F, suggesting that the clonogenic cells from children with AML are more readily assayed in AG/F. The number and size of clones in either system did not show a relationship to the morphologic subtype of leukemia. Depending on the scoring system used, increased growth in MC/P was related to abnormal karyotype. Also dependent on scoring system, the ability of leukemic cells to form small clusters in AG/F was associated with resistance to induction therapy: cells of patients with resistant disease were more likely to produce small clusters (p = 0.02). Our results suggest that clonogenic cells from children with AML grow more readily in AG/F than in MC/P, but that neither culture system supports the growth of cells from all patients. Depending on scoring criteria, in-vitro growth patterns in AG/F correlate with response to induction therapy.