Current evidence suggests that neonatal tolerance to a foreign Ag is the consequence of IL-4-mediated Th2 immunity rather than the thymic deletion of Ag-specific T cells. Here, we addressed the role of IL-4 in neonatal tolerance by testing whether tolerance to a minor histocompatibility Ag can be induced in newborn mice that lack IL-4 (IL-4−/−). We found that IL-4 does not play a dominant role in the induction of neonatal tolerance as newborn female IL-4−/− mice could be readily tolerized to the HY male Ag. In contrast, mice that lack both IL-4 and IL-13 (IL-4−/−/IL-13−/−) were resistant to the induction of neonatal tolerance, and their splenocytes produced exaggerated amounts of IFN-γ on rechallenge with the same Ag encountered during the neonatal period. These findings argue against the view that IL-4 alone is critical for the induction of neonatal tolerance and suggest that the combined actions of both IL-4 and IL-13 are essential for this process.