Gene cloning of rodent IgE-binding factors was accomplished, using messenger RN A of a rat T cell hybridoma, 23B6, which produce IgE-suppressive factor. Transfection of cos 7 monkey kidney cells with a eDNA clone resulted in the formation of rodent IgE-potentiating factors. The results provided a definitive evidence that IgE-binding factors represent a single peptide chain, and that the IgE-potentiating factor and IgE-suppressive factor share a common structural gene. Nucleotide sequence of a cDN A provided predicted amino acid sequence of the 60K precursor molecules of IgE-potentiating factor. Human IgE-potentiating factors were obtaiend from a human T-T hybridoma. The factors have affinity not only for human IgE but also for rat IgE and selectively enhanced antigen-induced IgE response of rat lymphocytes. The same hybridoma could be switched to form IgE-suppressive factor by the addition of glycosylation-inhibiting factor (GIF) during its biosynthesis. Purified GIF has immunosuppressive activity in the mouse. This factor suppressed the primary IgE and IgG antibody responses in the mouse, and markedly suppressed ongoing IgE antibody formation.