An in vitro colony assay was used to examine the growth stimulatory effects of a variety of recombinant human lymphohemopoietic cytokines on human precursor-B acute lymphoblastic leukemia (ALL) cells. Of 23 cases evaluated, 16 formed significant numbers of colonies (mean 280, range 36-939) when cultured in 10% fetal calf serum in 0.8% methylcellulose containing 10% partially purified B-cell growth factor (BCGF). Immunoperoxidase staining of cells from cultures confirmed a precursor-B phenotype (HLA-DR+, CD-10+, CD-19+, CD-34+, Ig-, CD-3-, CD-11C-). When these cases were cultured with recombinant human cytokines (but without BCGF) only a minority showed colony formation, in all instances less than seen with BCGF. Three cases were stimulated both by interleukin 3 (IL-3) and the putative pre-B growth factor interleukin 7 (IL-7). One case was stimulated both by tumor necrosis factor alpha and by interleukin 6 (IL-6); these results were confirmed on highly purified CD-10+, CD-19+ cells prepared by fluorescence-activated cell sorting. A further case was stimulated by granulocyte-macrophage colony-stimulating factor (GM-CSF), including CD-10+, CD-19+ purified cells. All cases responding to recombinant cytokines were heavily pretreated patients at relapse, whereas none of the newly diagnosed untreated cases showed any response. These results confirm that activities present in BCGF are the major stimulant for precursor-B ALL proliferation in vitro. None of the recombinant cytokines examined, including IL-7, appeared to have consistent activity under these culture conditions. The molecules regulating growth of ALL remain to be more precisely defined.