Immunological tolerance was induced in CBA mice with respect to both anti-hapten and anti-carrier IgE antibody production, following pretreatment of the animals with deaggregated ovalbumin. IgG antibody production was also affected. The tolerance was antigen-specific, was stable upon adoptive transfer to irradiated syngeneic recipients, but was reversed following booster. The extent and duration of the tolerant state depended on the dosage and number of tolerogen injections. Tolerogen administered after the initiation of the primary response was without effect. The pattern and duration of this tolerance suggested that T suppressor cells were not involved. The adoptive transfer of spleen cells from tolerogen-treated donors while being themselves unresponsive, failed to interfere with the induction of an immune response in the recipient. Evidence of T suppressor cell function was found in adoptive transfers, only after prolonged pretreatment of donors with a combination of tolerogenic and immunogenic forms of the carrier. These results suggested that T cell-dependent tolerance of the IgE antibody response operates via two distinct mechanisms, of which only one is provided by suppressor cell function.