The mechanism of selective anti-hapten IgE antibody production was studied in SJL mice. Using an adoptive transfer method of spleen cells into syngeneic recipients irradiated with a sublethal dose of 600 rads, it was demonstrated that for the suppression of anti-dinitrophenyl (DNP) IgE antibody production the interaction of two subsets of T cells is necessary. DNP-primed B cells and carrier-primed T helper cells are taken from donors primed with small amounts of DNP-carrier conjugates. Without injection of other cells, high titer and persistent anti-DNP antibodies are produced in the recipients. The two subsets of T cells that are active in suppression of IgE are taken from two types of donors: one donor is immunized (hyperprimed) with larger amounts of carrier protein twice, the other is an unprimed donor. The carrier for hyperpriming the first type of donor may be unrelated to the carrier used for priming the helper T cells. To bring about anti-DNP IgE suppression it is necessary that the animals should be challenged with the same DNP-carrier conjugate used for priming the B and T helper cells. If the hyperprimed donors were immunized with a heterologous, unrelated carrier, then this heterologous unconjugated carrier must also be injected together with the homologous DNP-carrier conjugate. In these conditions, anti-DNP IgE antibody production is suppressed, but the production of anti-DNP IgG1 antibody is not diminished.