Interactions between mammary epithelial and mesenchymal cells including fibroblasts and adipocytes are crucial for the proper postnatal development of the mammary ductal tree. Often overlooked, however, are the migrant cells that enter tissues at different stages of development. In this paper we identify two such cell types, macrophages and eosinophils, that are recruited around the growing terminal end buds (TEBs) during postnatal development. An important role for leukocytes in mammary gland ductal outgrowth is first demonstrated by depleting mice of leukocytes using sub-lethal γ-irradiation. This treatment results in a curtailment of mammary gland epithelial development that is completely rescued by bone-marrow transplantation, concurrent with a restoration of macrophage and eosinophil recruitment around the growing ducts. Using mice homozygous for a null mutation in the gene for CSF1 (Csfm^op/Csfm^op), the major growth factor for macrophages, we show that in the absence of CSF1, the population of macrophages in mammary glands is depleted. In this mutant, the formation of TEBs, their outgrowth into the fat pad and the branching of the resultant ducts are all impaired. Similarly, by using mice homozygous for a null mutation in the gene for eotaxin, a major chemokine for local recruitment of eosinophils in tissue, we identify eotaxin as the necessary and sufficient chemokine responsible for eosinophil recruitment around TEBs. In the absence of eosinophils, mammary gland branch formation and to a lesser extent TEB formation are reduced. Our data show that CSF1-regulated macrophages, in collaboration with eotaxin-regulated eosinophils, have essential and complementary functions in regulating the branching morphogenesis of the mammary gland.