The focus of this research was to investigate the use of an immortalized bovine mammary epithelial cell line as a starting material for xenogeneic transplantation into the mammary glands of immunocompetent recipients. PSG-5 cells (a clonal derivative of the MAC-T cell line engineered to express ovine IGF-I ) were transplanted into the cleared mammary fat pads of recipient mice. Following transplantation, spheroidal cell structures were observed in the cleared mammary fat pads of immunocompetent control mice and in mice exposed to PSG-5 cells during fetal development. Spheroidal cell structures were not observed in glands that had not received cell transplants. The success of this xenogeneic transplantation prompted the development of a MAC-T cell clone expressing a bacterial β-galactosidase (βMAC-T's) for use as a histological marker protein. Studies were then performed to determine the most appropriate age and location for cell transplantation into ovine recipients. Between three and 12 weeks of age, parenchymal volume in the mammary glands collected from e\ves in this study (n=4/age group: 3,6,9, and 12 weeks) increased nearly lO-fold (3.8cm³ to 34.5cm³ ). Total gland volume increased approximately 5-fold (67.3cm³ to 316.2cm³ ). Based on these determinations of parenchymal and glandular volumes, we determined that transplantation should begin with lambs at about three weeks of age. This data provides a starting point to begin trials using βMAC-T cells, which have been engineered to express a histological marker protein, for transplantation into intact ovine and murine mammary glands. / Master of Science