We found a tight correlation among the levels of H4/inducible costimulator (ICOS) expression, IL-4 production, and GATA-3 induction, using activated CD4+ T cells obtained from six different murine strains. BALB/c-activated CD4+ T cells expressed∼ 10-fold more H4/ICOS on their surfaces and produced∼ 10-fold more IL-4 upon restimulation than C57BL/6-activated CD4+ T cells. BALB/c naive CD4+ T cells were shown to produce much higher amounts of IL-2 and IL-4 upon primary stimulation than C57BL/6 naive CD4+ T cells. Neutralization of IL-4 with mAbs in culture of BALB/c naive CD4+ T cells strongly down-regulated both H4/ICOS expression on activated CD4+ T cells and IL-4 production upon subsequent restimulation. Conversely, exogenous IL-4 added to the culture of BALB/c or C57BL/6 naive CD4+ T cells up-regulated H4/ICOS expression and IL-4 production upon restimulation. In addition, retroviral expression of GATA-3 during the stimulation of naive CD4+ T cells from C57BL/6 or IL-4−/− mice increased H4/ICOS expression on activated CD4+ T cells. A similar effect of IL-2 in the primary culture of BALB/c naive CD4+ T cells appeared to be mediated by IL-4, the production of which was regulated by IL-2. These data suggest that IL-4 induced by IL-2 is critical to the maintenance of high H4/ICOS expression on BALB/c-activated CD4+ T cells.