Gene knockout techniques in the mouse have provided a unique opportunity to examine physiologic gene function. The phenotypes of mutant mice have resulted in a number of predicted as well as totally unexpected results, the latter perhaps providing the most valuable insight into otherwise unexplored biology. An excellent example to illustrate these points are gene knockout studies of interleukin (IL)-3, IL-5, granulocyte-macrophage colony-stimulating factor (GM-CSF), and the receptor system that cells use to bind these ligands. The role of IL-5 appears to be largely restricted to eosinophil biology, fulfilling expectations. However, the vivo function of GM-CSF, IL-3, and their receptor interaction has been more surprising. Neither cytokine nor their combination play an essential or even detectable role in primary bone marrow hematopoiesis, Instead, both IL-3 and GM-CSF play more specialized roles on certain cell subsets, basophil and mast cells, and alveolar macrophages, respectively. This review discusses recent findings with these mice, and based on these results, suggests a more appropriate view of the role of these cytokines and receptors in the hierarchy of molecular events leading to hematopoiesis.