Surface IgE-bearing (sIgE+) cells were studied in BN and rnu/rnu athymic rats after Nippostrongylus brasiliensis infection or i.p. injection of unpurified or purified IgE from plasmacytoma ascitic fluid. The number of sIgE+ cells increased markedly after a serum IgE increase without change in the proportion of sIgM+ and sIgD+ cells. A high percentage of the sIgE+ cells bore cytophilic IgE. Receptors for IgE were induced with a 2.56-micrograms IgE injection/100 g body weight and reached a maximum with 1.6 mg IgE/100 g body weight. They appeared less than 4 hr after a single injection of purified IgE. The number of IgE receptor-bearing cells reached a maximum plateau at 24 hr to day 3 after injection and declined thereafter, to reach the control level on day 9 or 11 after injection. Nearly all the sIgE+ cells of BN rats also bore sIgD, but the number of triple sIgE-sIgM-sIgD+ cells varied in a wide range. Maximum 4.5% of the sIgE+ cells of euthymic rats were T cells. More than 98% of the sIgE+ cells of nude rats were triple sIgM-sIgD-sIgE+ cells, and the majority were cytophilic IgE+. For the most part, the sIgM-sIgD-sIgE+ cells are probably not cells that can differentiate, as generally accepted, in IgE-producing cells. New interpretations of the role of these triple sIgM-sIgD-sIgE+ cells in IgE immune responses are necessary.