The hematopoietic cytokines granulocyte-macrophage colony-stimulating factor (GM-CSF), Interleukin (IL)-5 and IL-3 utilise a common receptor signalling molecule, the beta common chain (beta c). This shared receptor component explains, in part the overlapping actions of these cytokines. Mice lacking beta c have a low circulating eosinophil level, have impaired eosinophilic responses to parasitic infection and develop lung disease analogous to human pulmonary alveolar proteinosis (PAP). Surprisingly however, mature hematopoietic cell function is relatively intact, although all GM-CSF-mediated mature cell responses, including glucose transport are absent. Intriguing observations suggesting altered susceptibility to some infectious agents and amelioration of responses to inflammatory stimuli, require further clarification.