A number of alternatively spliced isoforms of haemopoietic growth factor receptors (HGFRs) have been described, but their role in human haemopoiesis remains undetermined. We have investigated the relative expression of the alpha1 and alpha2 isoforms of human granulocyte/macrophage colony-stimulating factor receptor (hGM-CSFR) during haemopoietic cell differentiation, and have shown that both subunits are independently regulated during differentiation of CD34+ human haemopoietic progenitor cells. To further investigate these ex-vivo observations, we established a series of murine FDCP mix cell lines, which, as a consequence of the ectopic expression of alpha1 or alpha2 hGM-CSFR, demonstrated differential differentiation responses to hGM-CSF. In this model system, hGM-CSFR-alpha2-expressing cells showed increased hGM-CSF-mediated erythroid/megakaryocytic differentiation compared with hGM-CSFR-alpha1-expressing cells.