The interleukin-5 (IL-5) receptor is a heterodimer that consists of an IL-5 specific alpha subunit and a common ssc chain that is shared with the receptors for granulocyte macrophage colony stimulating factor (GM-CSF) and interleukin-3 (IL-3). In contrast to IL-5, which acts mainly as an eosinophil lineage specific factor in vivo, IL-3 and GM-CSF stimulate the survival, proliferation and development of various hematopoietic cell lineages and also multipotent progenitor cells. IL-5 has little effect on the survival or proliferation of the multipotent stem cell line FDCP-Mix A4 but does promote some eosinophil development. To investigate whether the lineage specificity of IL-5 is due to the restricted expression of the IL-5 receptor alpha subunit we transfected the FDCP-Mix A4 cells with a retroviral vector containing this alpha subunit. The ectopic expression of the IL-5 receptor alpha subunit in the FDCP-Mix cells did not increase the observed eosinophilic development but did stimulate survival and proliferation of the transfected cells when IL-5 was added. IL-5 thus acts like IL-3 in these cells, promoting proliferation and survival. The results suggest that IL-5, whilst having a capacity to promote proliferation, does not influence eosinophilic lineage commitment in these multipotent cells. The results further argue that the observed lineage specificity of IL-5 is probably due to factors in addition to the restricted expression of the IL-5 receptor alpha subunit.