Cytokeratins are intermediate filament proteins found in most epithelial cells including the mammary epithelium. Specific cytokeratin expression has been found to mark different epithelial cell lineages and also to associate with putative mammary stem/progenitor cells. However, a comparative analysis of the expression of cytokaratins during embryonic and postnatal mammary development is currently lacking. Moreover, it is not clear whether the different classes of putative mammary stem/progenitor cells exist during embryonic development. Here, we use double/triple-label immunofluorescence and immunohistochemistry to systematically compare the expression of cytokeratin 5 (K5), cytokeratin 6 (K6),cytokeratin 8 (K8), cytokeratin 14 (K14) and cytokeratin 19 (K19) in embryonic and early postnatal mouse mammary glands. We show that K6+and K8+/K14+putative mammary progenitor cells arise during embryogenesis with distinct temporal and spatial distributions. Moreover, we describe a transient disconnection of the expression of K5 and K14, two cytokeratins that are often co-expressed, during the first postnatal weeks of mammary development. Finally, we report that cytokeratin expression in cultured primary mammary epithelial cells mimics that during the early stages of postnatal mammary development. These studies demonstrate an embryonic origin of putative mammary stem/progenitor cells. Moreover, they provide additional insights into the use of specific cytokeratins as markers of mammary epithelial differentiation, or the use of their promoters to direct gene overexpression or ablation in genetic studies of mouse mammary development. © The Author(s) 2009. This article is published with open access at Springerlink.com.