Mammary epithelium and surrounding stroma have been maintained in an explant system for 1 to 6 months and subsequently xenografted into athymic nude mice. The morphological characteristics of 26 cases of normal human mammary epithelium in long-term explant culture were described, using high-resolution light and electron microscopy. Normal human breast tissue specimens were obtained from immediate autopsy or surgical resection. The explants were cultured in Connaught Medical Research Laboratories Medium 1066 supplemented with serum, insulin, and hydrocortisone. The histotypic features of the mammary epithelium in both the central portion of the explant and the epithelial outgrowth onto the surface of the explant were described. In some cultures, the cells acquired more keratin and formed multicellular blister-like domes. Tissues from eight cases after 1 to 14 weeks in culture were xenografted in athymic nude mice and were maintained up to 924 days.