We used retroviral mediated gene transfer and gene knockout technologies to explore the in vivo functions of murine granulocyte-macrophage colony-stimulating factor (GM-CSF) [1, 2]. In tumor vaccination experiments, GM-CSF was the most potent molecule of a large number of cytokines, adhesion molecules and other immunomodulators for the induction of specific and long-lasting anti-tumor immunity. Vaccination required activities of both CD4 and CD8 positive lymphocytes, and likely involved the augmentation by GM-CSF of host professional antigen-presenting cell function. Mice engineered by homologous recombination techniques in embryonic stem cells to lack GM-CSF demonstrated no significant perturbations in steady-state hematopoiesis. All mutant animals, however, developed the accumulation of surfactant proteins and lipids in the alveolar space, the defining feature of the idiopathic human disorder pulmonary alveolar proteinosis. Surfactant lipid and protein content were increased in the absence of alterations in surfactant protein mRNA, supporting the concept that surfactant clearance or catabolism was perturbed. Extensive lymphoid hyperplasia associated with lung airways and blood vessels was also found, yet no infectious agents could be isolated. These results demonstrate that GM-CSF is not an essential growth factor for basal hematopoiesis and reveal an unexpected, critical role for GM-CSF in pulmonary homeostasis. It is tempting to speculate that the ability of GM-CSF to modulate the uptake and processing of particulate material underlies the mechanisms of immunostimulation and surfactant accumulation.