Apoptosis (programmed cell death) regulates cell population size. To determine the mechanisms whereby hematopoietic growth factors (HGFs) modulate apoptosis in human myeloid leukemic cells, we evaluated the roles of protein and mRNA synthesis for altering apoptosis in growth factor-stimulated vs. quiescent leukemic TF1 cells. Lysates of cells from the granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent myeloid leukemic cell line TF1 were separated into high molecular weight (HMW) pellets of intact DNA and supernatants of fragmented low MW (LMW) DNA, and the DNA purified from these fractions was quantified. In the absence of both GM-CSF and fetal bovine serum (FBS), 70% of the DNA was fragmented after 3 days in culture, with a characteristic apoptotic ladder-like pattern on agarose gel electrophoresis, whereas this proportion had initially been < 5%. In contrast, less than 5% of the DNA was fragmented in cells incubated with GM-CSF plus FBS or GM-CSF alone. Delayed addition of GM-CSF, but not FBS, permitted partial rescue of the cells, inhibiting increasing rates of accumulation of fragmented DNA. When the macro-molecular synthesis inhibitor cycloheximide (CHX) or actinomycin D (Act D) was present for 26 hours in the absence of GM-CSF and FBS, apoptosis was inhibited. In contrast, in the presence of GM-CSF or FBS, apoptosis was enhanced upon addition of CHX or Act D. The latter effect persisted even with the late addition of CHX. These findings indicate that disparate mechanisms of enhancing or inhibiting apoptosis exist in myeloid leukemic cells related to environmental conditions, including HGF-regulated cellular synthesis of distinct proteins and mRNA.