By the use of the adoptive transfer system in syngeneic mice it was demonstrated that: i) spleen cells of animals tolerized by the i.v. injection of DNP8-MgammaG 1 month before adoptive transfer into x-irradiated recipients remained unresponsive when further challenged with the sensitizing antigen (DNP2.3-OA administered i.p. with Al (OH)3); by contrast, a significant response (typical of a secondary response) was obtained in recipients of cells from mice which had received only the sensitized antigen; ii) the anti-hapten IgE secondary response of primed cells was not affected by the presence of cells of tolerized mice when these two types of cells were administered and challenged together in x-irradiated recipients; iii) the transfer of spleen cells or serum of mice, tolerized 20 days before harvesting, into intact syngeneic recipients did not interfere with the normal development of the anti-DNP IgE response of the latter; iv) the anti-hapten responses of tolerized animals receiving spleen cells from normal or primed mice together with the sensitizing antigen resulted, respectively, in a primary or secondary response; (v) the unresponsive state of tolerized cells, achievkd by immunosuppression of the original donors with respect to the haptenic determinant, was maintained even after two serial exposures of the cells to the immunizing antigen in two consecutive adoptive cell transfers into x-irradiated recipients. These findings provide a strong basis for the interpretation that the hapten-specific tolerace induced in mice by treatment with DNP8-MphiG involved the elimination or inactivation of hapten-specific IgE-producing cells or the blockade of the receptors of these cells.