Spleen cells of BALB/c mice were chemically modified with phosphorylcholine or benzylpenicilloyl hapten. The i.v. administration of such cells into syngeneic animals suppressed the formation of specific IgE antibodies against the respective hapten. The IgE antibody response against ovalbumin, which was used as an immunogenic carrier for the haptens, was not affected and the anti-hapten IgG or IgG1 response remained at the levels of the controls. The suppression could be transferred into X-irradiated mice by T cells from tolerized animals. Moreover, it was demonstrated that not only the induction of IgE, but also an established anti-hapten IgE antibody response is accessible to suppression by treatment with hapten-modified spleen cells from syngeneic animals. The results indicate that the i.v. administration of antigen coupled to syngeneic spleen cells induces T cells which suppress the formation of specific IgE antibodies in the primary and the secondary response without significantly affecting the formation of IgG antibodies.