The effects of granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-3 and IL-6 on clonogenic growth of blast-cell progenitors from 19 immunologically defined CD10-positive B-lineage acute lymphoblastic leukemias (ALL) coexpressing (My+ALLs) or not (My-ALLs) myeloid antigens have been studied. Our results demonstrate that GM-CSF was able to support the clonogenic growth of blast cells from My+ALLs, being totally ineffective on My-All samples. Accordingly, both alpha and beta chains of GM-CSF receptor (R) were expressed by My+ALL blasts, as investigated by reverse-transcriptase polymerase chain reaction (RT-PCR). Colony cells from GM-CSF-stimulated My+ALL cultures displayed the same immunophenotype as primary leukemic cells at diagnosis (CD10+, CD19+, CD22+), and retained the expression of myeloid-associated antigens and of GM-CSF-R transcripts. Moreover, My+ALL blasts showed a preferential sensitivity to the growth-promoting activity of IL-3 and IL-6, as compared with My-ALL cells. In addition to rearrangements of the JH region of immunoglobulin genes, My+ALL cells showed aberrant rearrangements of gamma (three cases) and beta (two cases) T-cell receptor genes, as well as of bcr sequences (three cases). Our data, showing an unexpected cross-lineage response of My+ALLs to GM-CSF, and their preferential stimulation by IL-3 and IL-6, as compared with My-ALLs, further support the concept that My+ALLs represent a separate entity with unique biological features.