When B cells are deprived of signaling through CD40, they exhibit the ability to induce T cell tolerance. The in vivo administration of anti-gp39 and allogeneic B cells diminished the ability of mice to mount an allogeneic response. Tolerance induction was specific for the haplotype expressed on the allogeneic B cells. Selective allospecific unresponsiveness was induced in the CD8 and CD4 compartments by the administration of anti-gp39 and class II-deficient B cells or class I-deficient B cells, respectively. As predicted by studies with anti-gp39 treatment, diminished allospecific responsiveness was induced by the administration of B cells to mice genetically deficient in gp39. Taken together, these data are consistent with the premise that deprivation of CD40 signaling engenders B cells with enhanced tolerogenicity. These studies provide insights into the tolerogenic capacity of resting B cells and outlines a practical approach to exploit this function.