Objective The c-myb is an important transcription factor in early haemopoietic system development and differentiation. We wish to use ES cell culture system by gene target getting cell models of c-myb+/+ and c-myb-/- ES in vitro in order to examine the detailed roles of the transcription factor c-myb in haemopoietic commitment and differentiation. Methods Haemopoietic differentiation was initiated by seeding ES cells in methylcellulose medium in the absence of LIF. The embryoid bodies of c-myb+/+ and c-myb-/- ES were analyzed by methylcellulose colony assay and real-time PCR to compare the formation of each haemopoietic system on different differentiation stage and procedure and relative gene expression. Results The formation of embryoid bodies was similar for both c-myb+/+ and c-myb-/- ES cells with the exception that the size and frequency of EBs reduced in the case of the c-myb-/- cells. The number of embryoid bodies increased more markedly in c-myb+/+ than that in c-myb-/-. Erythroid progenitors (CFU-E) were first present in c-myb+/+ group at 6 d after in vitro differentiation, and their number reached the peak at 8 d. Similar kinetics were seen for the formation of CFU-Es in c-myb-/- group, but their number was lower than that in c-myb+/+ group, and the colonies were generally small. CFU-M were first detectable at 7 d, and the peak levels were present at 7 d in c-myb+/+ group. Similar kinetics were seen for the formation of CFU-M in c-myb-/- group, but their number was lower than that in the c-myb+/+ group. CFU-GM was first detectable at 7 d, and the peak levels were present at 12 d in c-myb+/+ group. CFU-GM was not detected in c-myb-/- group. Real-time PCR analysis showed that there was no change in the gene expressions of ?-globin, zeta-globin, Lys, and C-fms in the two groups of c-myb+/+ and c-myb -/-. Conclusion Low levels of c-myb are suff-icient to allow progenitor expansion, but progression of progeniters towards terminal differentiation is significantly altered. Low levels of c-myb can influence erythropoiesis development, but precursor cells capable of differentiating into macrophages are present in haemopoietic commitment. The levels of c-myb can not change the gene expressions of ?-globin, zeta-globin, Lys, and C-fms.