Flow cytometric study revealed that almost all CD34⁺ cells in human umbilical cord blood expressed interferon‐γ receptor (IFN‐γR). To clarify the precise functional roles of IFN‐γR in human CD34⁺ cells, we examined the effect of IFN‐γ alone and in combination with various cytokines on the growth of haemopoietic progenitor cells in CD34⁺ cells using a serum‐free clonal culture. Surprisingly, IFN‐γ alone supported only megakaryocyte (MK) colonies in a dose‐dependent manner with a plateau level at 1000 U/ml of IFN‐γ. IFN‐γ at 1000 U/ml induced 10 ± 1.2 MK colonies from 1 × 10³ CD34⁺ cells, whereas thrombopoietin (TPO), interleukin (IL)‐3, stem cell factor (SCF) or IL‐6 alone induced 22 ± 4.0, 22 ± 4.2, 4 ± 0.6 and 0 MK colonies, respectively. The addition of anti‐IFN‐γ monoclonal antibody (mAb) to the IFN‐γ culture completely abrogated MK colony formation, whereas the mAb had no effect on TPO‐dependent production of MK colonies. In contrast, although anti‐TPO polyclonal Ab almost completely blocked TPO‐dependent MK colony formation, it failed to inhibit the generation of MK colonies induced by IFN‐γ, suggesting that the observed effect of IFN‐γ on the proliferation of human MK progenitor cells is independent of TPO. The addition of IFN‐γ to culture with TPO or SCF significantly augmented the development of MK colonies, whereas it did not affect IL‐3‐dependent MK colony formation. Additionally, IFN‐γ induced the increase of DNA content of cultured glycoprotein IIb/IIIa‐positive megakaryocytes. These results suggest that IFN‐γ may have regulatory roles in human megakaryocytopoiesis.