GADD153/CHOP is a DNA damage-inducible, nuclear leucine zipper protein that is capable of producing a G₁-S arrest in fibroblastic cells and of dimerizing with and inhibiting CAAT/enhancer binding protein (C/EBP) activities. CHOP was induced in 32D cl3 myeloid cells exposed to methylmethane sulfonate (MMS), a DNA alkylating agent. The degree of induction was dependent upon the dose of MMS to which the cells were exposed. CHOP was not expressed, at least at similar levels, during 32D cl3 cell granulocytic differentiation or during their apoptosis upon growth factor withdrawal. High-level expression of exogenous CHOP in 32D cl3 cells markedly inhibited the rans-activation activities of endogenous C/EBPs. These cells proliferated in IL-3, although low-level ongoing apoptosis not observed with control cells was detected. When these cultures were transferred to granulocyte colony-stimulating factor (G-CSF), the majority of the cells underwent apoptosis, although the levels of CHOP did not increase. Similarly, 32D cl3 cells treated with doses of MMS sufficient to induce endogenous CHOP underwent apoptosis more rapidly when placed in G-CSF-containing, compared with interleukin 3 (IL-3)-containing, medium. However, induction of CHOP by MMS was similar in IL-3 and in G-CSF. The heightened sensitivity of 32D cl3 cells to CHOP in G-CSF could result either from the loss of IL-3-specific signals or from increased expression of C/EBPs. Because myeloid leukemias express C/EBPα, induction of CHOP might contribute to their chemotherapy-induced apoptosis, and alterations in CHOP expression could contribute to their development of chemotherapy resistance.