BACKGROUND: The CCAAT/enhancer binding protein epsilon (C/EBPepsilon), one of the transcription factors, plays an important role in granulopoiesis. We examined an essential site of C/EBP epsilon for granulopoiesis. METHODS: 32Dcl3 cells and #1111 cells were transduced with retroviral constructs of C/EBP epsilon and C/EBP epsilon(R211A) (DNA binding mutant form). We examined growth rate, checked the neutrophil markers of Gr-1 and Mac-1, and counted differentiated cells in the transduced 32Dcl3 cells. The transduced #1111 cells were injected into five mice and survival times were analyzed. RESULTS: The mean number of green fluorescent protein (GFP) (+) 32Dcl3 cells transduced with C/EBP was 244, 045 at day 2, 582, 938 at day 4, and 873, 963 at day 6; mean expression of Gr-1 was 31.3% and Mac-1 32.6%; mean count of immature form was 41.0%, intermediate form 48.3%, and mature form 10.7%. In case of C/EBP epsilon (R211A) transduced 32Dcl3 cells, the respective figures were 707, 226, 1, 106, 736, and 2, 133, 819; 0.1% and 0.1%; and 91.7%, 4.3%, and 4.0%. The mean survival time of #1111 cells transduced with C/EBP epsilon was 26.0 days in placebo group and 34.0 days in 4-hydroxytamoxifen (4HT; C/EBP epsilon group); in case of C/EBP epsilon(R211A) transduced #1111 cells, the respective figures were 36.4 and 37.4 days. CONCLUSIONS: The growth of 32Dcl3 and #1111 cells was inhibited by C/EBP epsilon, but not by C/EBP epsilon(R211A). Also C/EBP epsilon was involved in the differentiation of 32Dcl3 cells, but C/EBP epsilon(R211A) was not. The DNA binding domain of C/EBP epsilon is a very important site for differentiating and inhibiting early myeloid cells.