The human AML-193 cell line requires exogenous granulocyte-monocyte colony-stimulating factor (GM-CSF) or interleukin-3 (IL-3) for growth in liquid or semisolid medium. However, these CSFs do not stimulate the differentiation of the cell line. We show that addition of all-trans retinoic acid (RA) or 1,25 dihydroxyvitamin D3 (D3) induces AML-193 cells to differentiate into the granulocytic or monocytic lineage, respectively. On the other hand, addition of either G- or M-CSF alone exerts virtually no differentiative effect. Terminal granulocytic or monocytic differentiation was observed when AML-193 cells were treated with RA and G-CSF, or D3 and M-CSF, respectively, as evaluated by cell morphology, analysis of surface antigens, and phagocytic functions. These positive interactions indicate that the differentiating activity of G- and M-CSF on leukemic cells may be unmasked by preliminary treatment with RA and D3, respectively, ie, the physiologic inducers override the leukemic differentiation blockade and CFSs exert their differentiative activity on the unblocked leukemic cells. These preliminary observations on a single cell line may pave the way for the designing of clinical protocols combining physiologic inducer(s) and hematopoietic growth factor(s) in the treatment of acute leukemia.