Formation of reaginic antibodies in mice has been demonstrated by numerous investigators (1–4). These antibodies were shown to be fixed to homologous skin, being retained there for several days. They were also shown to be heat and mercaptoethanol sensitive. Thus in many of their biologic activities mouse reaginic antibodies resemble human IgE antibodies (5). Availability of genetically thymusless nude mice provided an opportunity to examine the dependence of mouse reaginic antibody formation on the presence of thymus cells. Nude mice (nu/nu) were shown to respond poorly to sheep erythrocytes in respect to formation of IgM and IgG antibodies (6–8). The immune response to sheep erythrocytes was at least partially restored by normal thymus grafts or by injection of normal thymus cells (9). Nude mice (nu/nu) and heterozygous (nu/-) derived from back-crosses to the BALB/c strain (3 to 5 generations) were obtained from Bomholtgaard, Ltd., Ry. Denmark. BALB/c mice (+/+) originated from Jackson Laboratories, Bar Harbor, Maine.