SUMMARYAfter immunization with male skin grafts, implants of isologous male mammary gland tissue are rejected by C57BL female mice. Thus mammary gland tissue apparently contains the antigen associated with the Y chromosome. Nevertheless, implants of male mammary tissue in nonimmunized female hosts are not only capable of survival but also produce glandular outgrowths. Further, these outgrowths are physiologically competent for as long as 11 weeks after implantation; they can be stimulated to further development by the administration of hormones. Possible explanations for the growth of this male tissue in the female host are discussed.