The response of neoplastic basophil/mast cell precursors to various hematopoietic factors was examined. Blastic or promyelocytic immature cells were obtained from six patients in basophilic crisis of chronic myelogenous leukemia. In all cases, after 14 days suspension culture more then 90% of the cells had basophilic features. 3H-thymidine uptake was markedly increased by the addition of GM-CSF in two cases, G-CSF in one, and IL-3 in two. In clonogenic cell assays, numerous colony formations were obtained when using the same growth factors as in the 3H-thymidine uptake assay. In addition, IL-3 induced colony formation in one case, despite a lack of thymidine uptake IL-4 had a synergistic effect on colony formation with IL-3 in one other case. None of the factors used showed any effect on differentiation. These findings indicate that the proliferation of neoplastic basophil/mast cell precursors may be regulated by various growth factors but response patterns are divergent.