SJL mice are known for their poor IgE production upon helminth infection. In this study, we have demonstrated that SJL standard B cells (85% IgM+ or B220+), prepared by complement-mediated T cell lysis, failed to proliferate and to produce IgE and IgG1 in response to LPS plus IL-4 in vitro. This diminished IgE production was restored by anti-IL-12 and enhanced by additional treatment with anti-IL-18, suggesting active suppression by the cells that produce IL-12 and IL-18. Indeed, SJL standard B cells were contaminated with Mac-1+ cells. Therefore, we removed macrophages by passing standard B cells through a Sephadex G-10 column (G10). Resultant cells (95% IgM+), designated as G10-B cells, responded to LPS and IL-4 by their proliferation and differentiation. G-10 treatment markedly diminished the proportion of B220− cells and Mac-1+ cells in SJL standard B cells. Furthermore, addition of SJL B220− cells dose dependently and MHC independently inhibited LPS plus IL-4-induced B cell growth and IgE production in SJL and BALB/c B cells. B220− cells in SJL standard B cells contained Mac-1+ cells (51%) and Fas ligand+ CD4− CD8− double-negative CD3 int IL-2Rβ+ T cells (26%). Thus, IL-12 and IL-18 produced by LPS-stimulated Mac-1+ cells stimulate this unique subpopulation of T cells to produce IFN-γ, which in combination with Fas ligand, inhibits IgE production from the B cells. Our present results indicate that Mac-1+ cells and double-negative CD3 int IL-2Rβ+ T cells, uniquely abundant in the spleens of SJL mice, inhibit IgE production, indicating their new role in IgE response.