Changes during mammary gland development can be detected with methods using specific antibodies directed against specific cell structures. In the present study, the expression pattern of a type of intermediate filament called cytokeratins (CKs) was evaluated in tissue samples from mice mammary glands during the first five weeks post-partum (pp). Several CKs were analyzed with specific markers and immunohistochemistry methods. It is known that CK expression varies with physiological and pathological changes, and it has been reported to mark different epithelial cell lineages; its evaluation is therefore of considerable importance for studies of breast cancer of a stem/progenitor cell origin, both in humans and animals. Our trial provides additional knowledge relative to the use of specific antibodies and techniques as valuable tools to detect CKs during early pp mice mammary gland development (weeks 1 to 5 pp), emphasizing the role of CKs as markers of mammary epithelial differentiation.