To explore the hematopoiesis inhibition mechanisms of interferon-gamma (IFN-gamma), the effects of IFN-gamma on the expression of the cyclin D in the umbilical cord blood hematopoietic stem/progenitor cells were observed. In the experiments the CD34(+) cells were isolated from the cord blood with MIDI-MACS system; semi-solid methylcellulose culture technique was used to measure the formation of CFU-GM; the expression levels of cyclin D isoforms were assayed by semi-quantitative RT-PCR, after the hematopoietic stem/progenitor cells were incubated with IFN-gamma. The results indicated that IFN-gamma could inhibit the formation of CFU-GM and down-regulate the expression of cyclin D2 and cyclin D3 at the mRNA level. It is concluded that the IFN-gamma could inhibit the proliferation of hematopoietic stem cells and down-regulate the expression of cyclin D, that may be one mechanism underlying the hematopoietic inhibition of IFN-gamma.