Administration of multiple injections of conjugates of ovalbumin (OA) and polyethylene glycol (PEG) or its monomethoxy derivative (mPEG) into mice which had been sensitized with 2,4-dinitrophenylated OA (DNP3-OA) abrogated both the anti-OA and anti-DNP IgE responses, in spite of additional injections of the sensitizing dose of DNP3-OA in the presence of A1(OH)3. Treatment of mice with OA-PEG in A1(OH)3 stimulated preferentially helper T cells, whereas injection of mice with OA-PEG in the absence of adjuvant elicited predominantly suppressor T cells. The unresponsive state of mice which had been treated 21 days earlier with OA-PEG could not be broken by the transfer of normal spleen cells and an additional sensitizing dose of DNP3-OA. Transfer of spleen cells from tolerized animals to normal mice dampened the capacity of the latter to mount both anti-DNP and anti-OA IgE responses; however, the suppressive effect of these cells was eliminated by treatment of the normal recipients with cyclophosphamide, which is a procedure known to inactive suppressor T cells, and hence it may be concluded that this effect was not due to the carryover of the tolerogen with the transferred cells. All these results provide strong support for the conclusion that the suppressor cells induced by the treatment of mice with OA-PEG and OA-mPEG conjugates belonged to a T cell subpopulation, and that the B cells of these mice were devoid of suppressive activity.