The fetoplacental allograft and the malignant neoplasm represent the only 2 biologic conditions in which antigenic tissue is tolerated by a seemingly intact immune system. Recent investigations in the fields of tumor and pregnancy immunology have resulted in the recognition of a variety of immunologic mechanisms which are common to both systems. These common denominators are reviewed and a thesis suggesting a common mechanism in the non-rejection of pregnancy and malignancy, based on the presence of embryogenic antigens in both systems, is presented.