The protracted IgE anti-ovalbumin (OA) response given by BDF1 mice was studied using an adoptive transfer model. Spleen cells taken from immunized BDF1 mice can produce IgE antibody in irradiated recipients without further overt antigenic challenge. Depletion of macrophages in active spleen cell suspensions did not diminish the capacity of the remaining cells to give an adoptive response. Evidently the cells subserving the adoptive response are not fully developed in donor mice until 4 weeks after immunization, since spleen cells removed at shorter intervals after immunization gave either no or weak adoptive responses. The production of IgE antibody in irradiated recipient mice is prevented if transferred B or T lymphocytes are treated in vitro with either gamma irradiation or mitomycin C, suggesting proliferation of both B and T lymphocytes is essential for the adoptive response to develop. However, the requirement for proliferation is only transient, since one IgE antibody production reached a steady state in the adoptive recipients, it manifested extreme resistance to high dose irradiation. Whole body irradiation of 800 and 1000 rad was without effect on sustained IgE production. This latter observation was valid for both intact mice which were irradiated 8 weeks after immunization and also for irradiated adoptively immunized mice. It is suggested that the IgE anti-OA antibody measured in serum of BDF1 mice several months after immunization with 1 microgram OA and 1 mg Al(OH)3 is the product of long-lived antibody secreting cells.