We have developed a tissue dissociation procedure for the mouse mammary gland whereby it is possible to isolate the parenchyma as intact structural subcomponents essentially free of mesenchyme. Whole mammary fat pads are coarsely minced and subjected to a limited collagenase digestion, with mechanical dissociation for 90 min at 37° C. The parenchyma is released as a mixture of multicellular organoids and monodispersed cells. Use of a graded series of filters (400, 250, 150, 95, and 51 μ pore size) allows the separation of parenchyma from nonparenchymal material, with a further enrichment of the former into ductal, ductal-lobular, and terminal end-bud or alveolar populations. Yield is variable and dependent upon the nature of the starting tissue, e.g., mouse strain, age, and parity. These organoid fractions may be established separately in culture. This procedure allows for the study of the ductal and terminal regions of the mouse mammary gland in culture at the tissue level.