High doses of aqueous protein antigens induce a form of immunological tolerance in which interleukin 2 (IL-2)- and interferon gamma (IFN-gamma)-secreting T helper type 1 (Th1) cells are inhibited, but IL-4-secreting (Th2) cells are not. This is manifested by reduced proliferation of antigen-specific T cells upon in vitro restimulation, and marked suppression of specific antibody responses of the immunoglobulin (Ig)G2a, IgG2b, and IgG3 isotypes, but not of IgG1 and IgE. The role of the immunomodulatory cytokine IL-4 in this model of unresponsiveness to protein antigens has been examined. Administration of tolerizing antigen itself primes splenic CD4+ T cells for secretion of lymphokines, both IL-2 and IL-4. Neutralization of IL-4 in vivo with the anti-IL-4 antibody 11B11 during tolerance induction augments IFN-gamma production by T cells of tolerant mice, and reverses the suppression of IgG2a, IgG2b, and IgG3. This blockade of IL-4 function does not, however, restore the proliferative responses of T cells, suggesting that reduced T cell proliferation is due to direct T cell inactivation or anergy. Inhibiting the activity of IL-4 in vivo also inhibits the expansion of antigen-specific Th2-like cells, which are resistant to the induction of unresponsiveness. Thus, the immunologic consequences of high-dose tolerance are due to a combination of clonal T cell anergy and IL-4-mediated immune regulation.