The ontogeny of the immune response has usually been studied in intact neonatal animals or in cells isolated from neonatal animals (1–4). These studies have demonstrated an age-dependent emergence of immune responsiveness. Lymphoid cells isolated from rats soon after birth have a reduced capacity to respond to mitogens or to alloantigens in mixed leukocyte culture (MLC)3 (5, 6). Mechanisms controlling the development of immunologic responsiveness in lymphoid cell populations are poorly understood. Extracellular materials may influence the reactivity of lymphoid cells. For example, factors present in normal serum are potent inhibitors of MLC (7, 8). Many of these serum factors have been isolated from the α-globulin fraction (9); one of these, purified α-fetoprotein, which is the first major serum protein to appear in the embryo, has exhibited immunosuppressive capacity in MLC and mitogen studies (10–12).