Susceptibility to severe lupus in MRL-Faslpr mice requires not only the lpr mutation but also other predisposing genes. Using (MRL-Faslpr × B6-Faslpr)F2 (where B6 represents C57BL/6) intercrosses that utilize the highly susceptible MRL and poorly susceptible B6 backgrounds, we previously mapped CFA-enhanced systemic lupus-like autoimmunity to four loci, named Lmb1–4, on chromosomes 4, 5, 7, and 10. In the current study, we generated and analyzed reciprocal interval congenic mice for susceptibility to CFA-enhanced autoimmunity at all four Lmb loci. Although all loci had at least a slight effect on lymphoproliferation, only Lmb3 demonstrated a major effect on lymphoproliferation and anti-chromatin Ab levels. Further characterization of Lmb3, primarily by comparing MRL-Faslpr with MRL.B6-Lmb3 Faslpr congenic mice, revealed that it also played a significant role in spontaneous lupus, modifying lymphoproliferation, IgG and autoantibody levels, kidney disease, and survival. The less susceptible B6 Lmb3 locus was associated with a marked reduction in numbers of CD4+ and double-negative (CD4−CD8−) T cells, particularly in lymph nodes, as well as reduced T cell proliferation and enhanced T cell apoptosis, both in vivo and in vitro. IFN-γ-producing CD4+ T cells were also reduced in MRL.B6-Lmb3 Faslpr mice. Further mapping using subinterval congenic mice placed Lmb3 in the telomeric portion of chromosome 7. Thus, Lmb3, primarily through its effects on CD4+ and double-negative T cells, appears to be a highly penetrant lupus-modifying locus. Identification of the underlying genetic alteration responsible for this quantitative trait locus should provide new insights into lupus-modifying genes.