The purpose of this study was to determine possible mechanisms for the recently observed association between insensitivity of acute myeloid leukemia (AML) clonogenic cells to colony stimulating activity (CSA) and poor response to induction chemotherapy. The bone marrow endogenous CSA was determined using semi-solid agar cultures by measuring the response of the AML patients' own clonogenic cells to endogenous CSA. The results show that whereas 31% (5/16) of patients at presentation have deficient bone marrow endogenous CSA production, over 50% (11/21) have relative deficiency of endogenous CSA, due to insensitivity of the patients' clonogenic cells to CSA. Although there is an association between relative deficiency of endogenous CSA and a poor response to therapy, the relationship is not close enough to explain the previously observed highly significant correlation between insensitivity to CSA and poor response to therapy. The CSA-insensitive phenotype and poor response to therapy, one via the tendency to relative endogenous CSA deficiency in the CSA-insensitive group and another via some additional feature of these poor response AML phenotypes which is independent of the presence or absence of endogenous CSA deficiency.