We have recently developed a model in which mice were treated with IL-4 after primary immunization, resulting in elevated total serum IgG1 and IgE levels, but decreased antigen-specific levels and memory formation for these isotypes. In this report, we describe that these effects of IL-4 are mediated at the B cell and not the T-cell level. Major changes occurred in the gamma 1 epsilon-double positive B-cell population which is increased as a result of IL-4 treatment. Moreover, it is shown that gamma 1 epsilon-double positive B cells can develop in vitro out of gamma 1-positive primed B cells and that these double positive cells can differentiate into IgG1- and IgE-secreting cells. The existence of gamma 1 epsilon-double positive memory B cells can explain the differences in cytokine dependence of TNP-specific memory IgG1 and IgE responses found after adoptively transferring primed spleen cells into irradiated naive recipients. Whereas the IL-4 independent TNP-specific memory IgG1 responses could be blocked efficiently by neutralizing IL-5 and IL-6, TNP-specific memory IgE responses were virtually not susceptible to such treatment. These IgE responses were also not susceptible to IFN-gamma, used in doses that could inhibit the primary IgE response. Inhibition of the TNP-specific memory IgG1 response by neutralizing IL-5 and IL-6 is accompanied by a 10-fold increase of the IL-4 independent TNP-specific IgE memory response. These data indicate that secondary IgE responses primarily result from B cells that are either switched to IgG1, or are double positive for IgG1 and IgE, thereby suggesting a minor role for epsilon-single positive B cells in secondary IgE responses.