We have studied the effect of recombinant human hematopoietic growth factors (interleukin-3 [rhIL-3], granulocyte-macrophage colony- stimulating factor [rhGM-CSF], and granulocyte CSF [rhG-CSF]) on the clonal growth of human colon adenocarcinoma cell lines HTB-38, CCL 187, and WiDr (CCL 218). The factors stimulated clonal growth of HTB-38 and CCL 187 in a capillary modification of the human tumor clonogenic assay in agar up to twofold. There were dose-response correlations over a range of 1 to 10,000 U/mL for rhIL-3, rhGM-CSF, and rhG-CSF. Incubation with neutralizing monoclonal antibodies abolished the stimulation of clonal growth by rhGM-CSF. The WiDr cell line was nonresponsive to rhIL- 3 and rhGM-CSF. These results represent the first evidence that a variety of hematopoietic growth factors can stimulate the growth of clonogenic cells of some nonhematopoietic malignant cell lines in vitro.